Atomic Structure and Chemical Bonding

Exercise 4(A)

Question 1

What is the contribution of the following in Atomic structure?

(a) Maharshi Kanada

(b) Democritus?

Answer

(a) The idea of the smallest unit of matter was given by Maharshi Kanada. According to him, matter consisted of indestructible minute particles called paramanus, now called atoms. A paramanus does not exist in free state, rather it combines with other paramanus to form a bigger particle called anu, now known as molecule. There are different types of paramanus. Each one of them exhibits specific properties.

(b) Democritus called the paramanu an atom, which is derived from the Greek word atomos, meaning indivisible.

Question 2

State Dalton's atomic theory.

Answer

John Dalton gave the first scientific theory about the structure of matter. He considered atoms as indivisible particles that are the fundamental building blocks of matter. The existence of different types of matter is considered to be due to different types of atoms constituting them.

Main postulates of Dalton's atomic theory are:

1. Matter consists of very small and indivisible particles called atoms.
2. Atoms can neither be created nor be destroyed.
3. The atoms of an element are alike in all respects, but they differ from the atoms of other elements.
4. Atoms of an element combine in small numbers to form molecules.
5. Atoms of one element combine with atoms of another element in a simple ratio to form molecules of compounds.
6. Atoms are the smallest units of matter that can take part in a chemical reaction.

Question 3

What is an α (alpha) particle?

Answer

An α (alpha) particle is a doubly charged helium ion (He²⁺) containing two protons and two neutrons. It is formed by removing 2 electrons from helium atom.

Question 4

What are the cathode rays? How are these rays formed?

Answer

Cathode rays are a stream of electrons that travel from the negatively charged electrode (the cathode) to the positively charged electrode (the anode) in a discharge tube.

Formation of cathode rays — When a high voltage (10,000 volts) charge from an induction coil is applied to tubes filled with gases at very low pressure (0.01 mm of Hg), the gases become good conductors of electricity and begin to flow from cathode to anode in the form of rays which are called cathode rays.

Question 5

What is the nature of a charge on

(i) cathode rays

(ii) anode rays?

Answer

Cathode rays are negatively charged as they are made of negatively charged particles.

Anode rays are positively charged as they are made of positively charged particles.

Question 6

How are X-rays produced?

Answer

When a beam of cathode rays is made to fall upon hard metallic targets like tungsten, X-rays are produced.

Question 7

Why were anode rays also called as 'canal rays'?

Answer

In discharge tube experiment, Goldstein noticed another set of rays travelling in a direction opposite to that of the cathode rays i.e., from anode towards cathode, when perforated cathode was used in the discharge tube. He called these rays as canal rays since these rays passed through the holes or canals in the cathode. These rays were named positive rays or anode rays.

Question 8

How do cathode rays differ from anode rays?

Answer

Question 9

State one observation which shows that atom is not indivisible.

Answer

Discovery of sub-atomic particles i.e., electrons, protons and neutrons shows that atoms are divisible.

In discharge tube experiment, Goldstein noticed, set of rays travelling in a direction opposite to that of the cathode rays. These were called canal rays since these rays passed through the holes or canals in the cathode. These rays were named positive rays or anode rays. These rays consist of sub-atomic particle proton.

Question 10

(a) Name an element which does not contain neutron.

(b) If an atom contains one electron and one proton, will it carry any charge or not?

Answer

(a) Hydrogen does not contain neutron.

(b) If an atom contains one electron and one proton, it will not carry any charge i.e., it will be a neutral atom.

Question 11

On the basis of Thomson's model of an atom, explain how an atom as a whole is neutral.

Answer

According to Thomson's atomic model —

1. An atom is considered to be a sphere of uniform positive charge and electrons are embedded into it.
2. The total positive charge is equal to the total negative charge so that an atom as a whole is electrically neutral.

Question 12

Which sub-atomic particle was discovered by

(a) Thomson

(b) Goldstein

(c) Chadwick.

Answer

(a) J. J. Thomson — Electrons

(b) E.Goldstein — Protons

(c) James Chadwick — Neutrons

Question 13

Name the sub-atomic particle whose charge is

(a) +1

(b) -1

(c) 0.

Answer

(a) Proton (+1)

(b) Electron (-1)

(c) Neutron (0)

Question 14

(a) Which metal did Rutherford select for his α-particle scattering experiment and why ?

(b) What do you think would be the observation of α-particle scattering experiment if carried out on

(i) heavy nucleus like platinum

(ii) light nuclei like lithium.

Answer

(a) Rutherford selected gold for his α-particle scattering experiment as he wanted as thin a layer as possible and gold is the most malleable metal.

(b) (i) Heavy nucleus like platinum will show the same observation with α-particles as shown by gold foil.

1. Most of the α-particle will pass straight.
2. Some α-particle will slightly deflect from their straight path.
3. Very few will be deflected by very large angles or completely bounce back.

(ii) If light nuclei like lithium is used then fast moving α-particles may even push the light nucleus aside and may not be deflected back.

Question 15

On the basis of Rutherford's model of an atom, which subatomic particle is present in the nucleus of an atom?

Answer

According to Rutherford's model of an atom, there is a positively charged mass in the centre of the atom, i.e., nucleus, in which the entire mass of the atom is concentrated, which makes nucleus the densest part of the atom. The electrically positive nature of the nucleus is supported by the fact that the positively charged α-particles approaching the nucleus get deflected. This proved that nucleus contained protons.

Question 16

Which part of an atom was discovered by Rutherford?

Answer

Rutherford discovered the nucleus.

Question 17

How was it shown that the atom has empty space?

Answer

Rutherford through an experiment, directed a stream of α-particles towards a very thin gold foil. Most of the α-particle pass through the metal foil without deviating from their path. This showed that an atom contains a large empty space (extra nuclear space).

Question 18

State one major drawback of Rutherford's model.

Answer

The comparison of electrons with planets in the solar system is the main drawback of Rutherford's model. It failed to explain the stability of the atom.

Question 19

In the figure given alongside :

(a) Name the shells denoted by A, B and C. Which shell has the least energy?

(b) Name X and state the charge on it.

(c) The above sketch is of ............... model of an atom.

Answer

(a) A is First energy level (K shell)

B is Second energy level (L shell)

C is Third energy level (M shell)

Shell K has the minimum amount of energy.

(b) X is the nucleus, it is positively charged.

(c) The above sketch is of Bohr's atomic model model of an atom.

Question 20

Give the postulates of Bohr's atomic model.

Answer

Postulates of Bohr's atomic model are as follows

1. The electrons revolving around the nucleus are confined to certain fixed orbits called shells or energy levels, each of which is associated with a fixed amount of energy.
2. While it is revolving around the nucleus in an orbit, an electron neither loses nor gains energy.
3. An electron revolving in a particular orbit, on gaining a certain amount of energy, jumps to the next orbit and vice verse. Since each orbit is associated with a fixed amount of energy, Bohr called it an energy level.

Exercise 4(B)

Question 1

(a) Name the three fundamental particles of an atom.

(b) Give the symbol and charge of each particle.

Answer

(a) The three fundamental particles of an atom are:

1. electrons
2. protons
3. neutrons

(b) The table below shows the symbol and charge of the particles:

Question 2

Complete the table given below by identifying P, Q, R and S.

Answer

P — 12

Q — 17

R — ²³⁸₉₂U

S — Fluorine

Question 3

The atom of an element is made up of 4 protons, 5 neutrons and 4 electrons. What are its atomic number and mass number ?

Answer

Atomic number (Number of protons or number of electrons) = 4

Mass number (Number of protons + Number of neutrons) = 4 + 5 = 9

Question 4

The atomic number and mass number of sodium are 11 and 23 respectively. What information is conveyed by this statement?

Answer

Atomic number of sodium is 11 implies that number of protons in the nucleus of sodium = number of electrons in a neutral atom of sodium = 11

Mass number of sodium is 23 implies that the sum of protons and neutrons in the nucleus of sodium = 23

Question 5

Write down the names of the particles represented by the following symbols and explain the meaning of superscript and subscript numbers attached.

₁p¹, ₀n¹, _-1e⁰

Answer

p = proton

n = neutron

e = electron

Question 6

From the symbol ²⁴₁₂Mg state the mass number, the atomic number and electronic configuration of magnesium.

Answer

Mass number = 24

Atomic number = 12

No. of electrons = 24 - 12 = 12

Electronic configuration = 2, 8, 2

Question 7

Sulphur has an atomic number 16 and mass number of 32. State the number of protons and neutrons in the nucleus of sulphur. Give a simple diagram to show the arrangement of electrons in an atom of sulphur.

Answer

Given, Atomic number = 16 and mass number = 32

So, Number of protons = 16

Number of electrons = 16

Number of neutrons = 32 – 16 = 16

Electronic configuration = 2, 8, 6

Below diagram shows the arrangement of the electrons in an atom of sulphur:

Question 8

Explain the rule according to which electrons are filled in various energy levels.

Answer

Distribution of electrons into various orbits was suggested by Bohr and Bury

1. The maximum possible number of electrons in a particular shell is given by the formula 2n², where n is the serial number of that shell.
   First shell or K shell has (2 x 1²) = 2 electrons
   Second shell or L shell has (2 x 2²) = 8 electrons
   Third shell or M shell has (2 x 3²) = 18 electrons
   Fourth shell or N shell has (2 x 4²) = 32 electrons
   and so on.
2. Electrons are not accommodated in a given shell unless the inner shells are filled. In other words the shells are filled in stepwise manner.
3. The outermost shell of an atom cannot accommodate more than 8 electrons, even if it has the capacity to accommodate more than 8 electrons.
4. The outermost shell of a chemically stable atom can accommodate a maximum of 8 electrons, except Hydrogen and Helium atoms, which have only one shell, and can thus accommodate a maximum of 2 electrons only.

Question 9

Draw the orbital diagram of ₂₀⁴⁰Ca⁺² ion and state the number of three fundamental particles present in it.

Answer

Below is the orbital diagram of ₂₀⁴⁰Ca⁺² ion:

Three fundamental particles of ₂₀⁴⁰Ca⁺² are:

1. Protons — 20
2. Electrons — 18
3. Neutrons — 40 - 20 = 20

Question 10

Write down the electronic configuration of the following:

(a) ₁₃²⁷X

(b) ₁₇³⁵Y

Write down the number of electrons in X and neutrons in Y.

Answer

(a) electronic configuration of ₁₃²⁷X is [2, 8, 3]

(b) electronic configuration of ₁₇³⁵Y is [2, 8, 7]

Number of electrons in X = 13

Mass number of Y = 35 and number of protons in Y = 17

∴ Number of neutrons in Y = 35 - 17 = 18

Exercise 4(C)

Question 1(a)

Rutherford's alpha-particle scattering experiment led to the discovery of :

1. Electron
2. Proton
3. Atomic nucleus
4. Neutron

Answer

Atomic nucleus

Reason — Rutherford through an experiment, directed a stream of α-particles towards a very thin gold foil. Most of the α-particles passed through the metal foil without deviating from their path. Very few (nearly one in ten thousand) alpha particles were either deflected by very large angles or completely bounced back. This suggested the presence of a positively charged mass at the centre of the atom, known as the nucleus.

Question 1(b)

The number of valence electrons in O^2- is :

1. 6
2. 8
3. 10
4. 4

Answer

8

Reason — Oxygen has atomic no. 8. Its electronic configuration becomes 2, 6 i.e., it has 6 valence electrons. In O^2-, there are two more electrons. Hence, number of valence electrons in O^2- is 8.

Question 1(c)

Which of the following is the correct electronic configuration of potassium ?

1. 2, 8, 9
2. 8, 2, 9
3. 2, 8, 8, 1
4. 1, 2, 8, 8

Answer

2, 8, 8, 1

Reason — Potassium has 19 electrons hence the electronic configuration is [2, 8, 8, 1]

Question 1(d)

The mass number of an atom whose unipositive ion has 10 electrons and 12 neutrons is :

1. 23
2. 22
3. 20
4. 21

Answer

23

Reason — As the atom is unipositive so the actual number of electrons must be 11 and mass number will be 11 + 12 = 23

Question 1(e)

The number of valence electrons in Cl^- ion is:

1. 16
2. 8
3. 17
4. 18

Answer

8

Reason — Chlorine has atomic number 17 and electronic configuration is 2, 8, 7. For Cl^-, number of electrons increases by 1 hence electronic configuration becomes 2, 8, 8 and valence electrons will be 8.

Question 1(f)

The correct electronic configuration of Fluorine is:

1. 2, 8
2. 8, 2, 1
3. 7, 2
4. 2, 7

Answer

2, 7

Reason — Fluorine has atomic number 9 and electronic configuration is 2, 7.

Question 1(g)

An element 'A' has 15 electrons and 16 neutrons. The correct representation of this element is:

1. ¹⁶₁₅A
2. ³¹₁₆A
3. ³¹₁₅A
4. ¹⁵₁₆A

Answer

³¹₁₅A

Reason — Atomic number is 15 so it will be written in subscript and mass number = 15 + 16 = 31 so it will be written in superscript. Hence the correct representation will be ³¹₁₅A

Question 1(h)

An atom having 3 protons and 4 neutrons will have a valency of :

1. 3
2. 7
3. 1
4. 4

Answer

1

Reason — Number of electrons = number of protons = 3, hence electronic configuration = 2,1. Hence, the valency is 1.

Question 1(i)

Elements with valency 2 are:

1. Always metals
2. Always non-metals
3. Can be metals or non-metals
4. Metalloids (semi metals)

Answer

Can be metals or non-metals

Reason — If the element shows positive valency it is a metal (e.g., Beryllium) and if the element shows negative valency it will be a non-metal (e.g., Sulphur).

Question 1(j)

Nucleus of an atom is composed of :

1. Electrons and neutrons
2. Protons and neutrons
3. Protons, neutrons and electrons
4. Electrons and protons

Answer

Protons and neutrons

Reason — The nucleus is located at the centre of the atom, it contains protons and neutrons, which account for the total mass of that atom.

Question 1(k)

There are 3 electrons in M shell of an atom. Its atomic number is:

1. 11
2. 13
3. 15
4. 3

Answer

13

Reason — M shell has 3 electrons so the electronic configuration becomes 2, 8, 3. Hence the total number of electrons = 13 and the atomic number becomes 13.

Question 1(l)

The maximum number of electrons that M shell can occupy is:

1. 18
2. 8
3. 2
4. 10

Answer

18

Reason — The maximum possible number of electrons in a particular shell is given by the formula 2n², where n is the serial number of that shell.
Third shell or M shell has (2 x 3²) = 18 electrons

Question 1(m)

Which orbit is nearest to the nucleus of an atom?

1. K
2. L
3. M
4. N

Answer

K

Reason — Orbits are arranged as K, L, M, N. So the orbit closest to the nucleus is K.

Question 1(n)

Atoms combine to attain an electronic configuration similar to their nearest :

1. Metals
2. Noble gas
3. Non-metals
4. Semi metals

Answer

Noble gas

Reason — Elements tend to combine with one another to attain the stable electronic configuration of the nearest noble gas i.e., 2 or 8 electrons in its valence shell.

Question 1(o)

Isotopes differ in the number of their:

1. Electrons
2. Protons
3. Neutrons
4. Electrons and protons

Answer

Neutrons

Reason — Isotopes are the elements having the same atomic number but different mass numbers.

For example : ²⁴₁₂Mg and ²⁶₁₂Mg

Isotopes differ in terms of neutrons.

Question 2

Name the following:

(a) The element which does not contain any neutron in its nucleus.

(b) An element having valency 'zero'.

(c) Metal with valency 2

(d) Two atoms having the same number of protons and electrons but different number of neutrons.

(e) The shell closest to the nucleus of an atom.

Answer

(a) Hydrogen

(b) Helium

(c) Magnesium

(d) Isotopes

(e) K shell

Question 3

Match the atomic numbers 4, 14, 8, 15 and 19 with each of the following :

(a) A solid non-metal of valency 3.

(b) A gas of valency 2.

(c) A metal of valency 1.

(d) A non-metal of valency 4.

Answer

Explanation

(a) As atomic number 15 has electronic configuration [2,8,5], so it is a non-metal having 5 electrons in the valence shell and needs three electrons to complete its octet so it has a valency 3.

(b) As atomic number is 8, so it has the electronic configuration of [2,6] and it needs 2 electrons to complete its octet hence, it is a gas of valency 2

(c) As atomic number 19 has electronic configuration [2,8,8,1], so it is a metal having 1 electron in the valence shell and donates its one electron to complete its octet so it has a valency 1.

(d) As atomic number 14 has electronic configuration [2,8,4], so it is a non-metal having 4 electrons in the valence shell and needs four electrons to complete its octet so it has a valency 4.

Question 4

(a) What are inert elements?

(b) Why do they exist as monoatoms in molecules?

(c) What are valence electrons?

Answer

(a) Elements with the valence shell having 8 electrons and helium with 2 electrons, cannot gain or lose electrons so they are chemically inactive and are called inert elements.

(b) Inert elements exist as monoatoms because they have a completely filled outermost electron shell. This makes them very stable and unlikely to form chemical bonds with other atoms. Hence, they tend to exist as monoatoms in molecules rather than participating in chemical reactions.

(c) Electrons present in the outermost shell (i.e., valence shell) of an atom are valence electrons.

Question 5

In what respect do the three isotopes of hydrogen differ ? Give their structures.

Answer

Hydrogen has three isotopes i.e., namely protium (¹₁H), deuterium (²₁H) and tritium (³₁H) each having the same atomic number i.e., 1 but different mass numbers i.e., 1, 2 and 3 respectively.

Hence, all three have same number of electrons and protons whereas number of neutrons are 0, 1 and 2 respectively.

Isotopes of Hydrogen

Question 6

What is the significance of the number of protons found in the atoms of different elements?

Answer

As elements are arranged on the basis of the number of protons in their respective atoms, therefore, proton helps in determining and understanding the structure and behavior of an atom. Protons also determine the positive charge of the nucleus in the atom

Question 7

Element X has electronic configuration 2, 8, 18, 8, 1. Without identifying X,

(a) predict the sign and charge on a simple ion of X.

(b) write if X will be an oxidizing agent or a reducing agent. Why?

Answer

(a) X⁺

Reason — Since the element X has 1 electron in its valence shell. So, it will lose an electron and gain noble gas configuration. Therefore it forms a cation (X⁺). The sign is positive and the charge is of 1 proton.

(b) X will be a strong reducing agent as it has the tendency to donate its valence electron.

Question 8

Define the terms :

(a) mass number

(b) ion

(c) cation

(d) anion

(e) element

(f) orbit.

Answer

(a) Mass number (A) — The mass number of an atom is equal to the total number of nucleons (i.e., the sum of the number of protons and the number of neutrons ) in it's nucleus. It is denoted by A.

A = number of protons + number of neutrons

Mass number however is a whole number approximation of the atomic mass calculated in atomic mass units.

(b) Ion — An ion is a charged particle which is formed due to the gain or loss of one or more electrons by an atom.

For example, a sodium ion (Na⁺) is formed when a sodium atom loses one electron and Chlorine ion (Cl^-) ion is formed when a chlorine atom gains an electron.

(c) Cation — A metallic atom, which loses electron(s), becomes a positively charged ion and is known as a cation.

(d) Anion — A non-metallic atom, which gains electron(s) becomes a negatively charged ion and is known as an anion.

(e) Element — An element is a pure substance that can neither be formed from, nor decomposed into simple substances by ordinary physical or chemical methods.

(f) Orbit — The imaginary circular path around the nucleus of an atom, in which the electrons revolve.

Question 9

From the symbol ⁴₂He for the element helium, write down the mass number and the atomic number of the element.

Answer

Atomic number = 2

Mass number = 4

Question 10

An atom of an element has two electrons in the M shell. What is the

(a) atomic number

(b) number of protons in this element?

Answer

(a) Number of electrons in the M shell = 2. So, the electrons in the K and L shells = 2 and 8

Hence, atomic number = 2 + 8 + 2 = 12

(b) As number of protons = number of electrons = 12

Question 11

Five atoms are labelled A to E.

(a) Which one of these atoms:

(i) contains 7 protons

(ii) has electronic configuration 2, 7?

(b) Write down the formula of the compound formed between C and D.

(c) Predict :

(i) metals

(ii) non-metals.

Answer

(a) (i) Atom E.

As its atomic number is given as 7, therefore it contains 7 protons.

(ii) Atom B.

As its atomic number is 9, hence it has 9 electrons, therefore, its electronic configuration is 2, 7.

(b) Atom C has electronic configuration : [2, 1]

Atom D has electronic configuration : [2, 6]

As D requires two electrons to complete its octet, hence, it combines with two atoms of C which donate their valence electrons to attain stable noble gas configuration and D accepts the two electrons and attains a stable state, hence the formula of the compound formed between C and D is C₂D

(c) (i) Metals: A [2,8,8,2] and C [2,1]
Reason — As the elements A and C have 2 and 1 electrons respectively in the valence shells hence, they are metals.

(ii) Non-metals: B [2,7], D [2,6], E [2,5].
Reason — As the elements B,D and E have 7,6 and 5 electrons respectively in the valence shell, hence they are non-metals.

Question 12

$^{24}_{12}\text{Mg}$ and $^{26}_{12}\text{Mg}$ are symbols of two isotopes of magnesium. Compare the atoms of these isotopes with respect to :

(i) the composition of their nuclei.

(ii) their electronic configurations.

(iii) give the reason why the two isotopes of magnesium have different mass numbers.

Answer

(i) The composition of their nuclei :

$^{24}_{12}\text{Mg}$ :

Atomic number [Z] = electron [e] = proton [p] = 12

Mass number [A] = 24

As, neutron [n] = A - Z = 24 - 12 = 12.

Hence, p = 12, n = 12

$^{26}_{12}\text{Mg}$ :

Atomic number [Z] = electron [e] = proton [p] = 12

Mass number [A] = 26

As, neutron [n] = A - Z = 26 - 12 = 14.

Hence, p = 12, n = 14

(ii) Their electronic configurations :

$^{24}_{12}\text{Mg}$ :

As electron [e] = 12 hence, electronic configuration = 2,8,2

$^{26}_{12}\text{Mg}$ :

As electron [e] = 12 hence, electronic configuration = 2,8,2

(iii) Reason for difference in mass numbers — As the number of neutrons is different in the two isotopes of magnesium, therefore, they have different mass numbers.

Question 13

What are nucleons? How many nucleons are present in phosphorus? Draw its structure.

Answer

Constituents of the nucleus are called nucleons, i.e. protons and neutrons.

³¹₁₅P has atomic number 15, hence it has 15 protons.

Mass number 31, hence number of neutrons = 31 - 15 = 16

Therefore, number of nucleons are p + n = 15 + 16 = 31.

Structure of Phosphorus is shown in the diagram below:

Question 14

What are isotopes? With reference to which fundamental particle do isotopes differ ? Give two uses of isotopes.

Answer

Isotopes are atoms of the same element having same atomic number but different mass number.

For example : ²⁴₁₂Mg and ²⁶₁₂Mg

Isotopes differ in the number of neutrons.

Uses of isotopes

1. Radioactive isotopes are used in industry to detect the leakage in underground oil pipelines, gas pipelines and water pipelines.
2. An isotope of Uranium ²³⁵₉₂U is used as a fuel in nuclear reactors.

Question 15

Why do ³⁵₁₇Cl and ³⁷₁₇Cl have the same chemical properties? In what respect do these atoms differ?

Answer

Isotopes have the same atomic number i.e., the same number of electrons and protons and hence the same electronic configuration. Chemical properties are determined by the electronic configuration of an atom. Thus, the isotopes of an element are chemically alike.

Isotopes have different mass numbers [A] due to different number of neutrons. ³⁵₁₇Cl has 18 neutrons whereas ³⁷₁₇Cl has 20 neutrons. Due to different atomic masses these atoms differ in their physical properties like density, melting point, boiling point, etc.

Question 16

Explain fractional atomic mass. What is the fractional mass of chlorine?

Answer

Fractional atomic mass is the weighted average of the atomic masses of all the naturally occurring isotopes of that element.

Fractional mass of chlorine is 35.5.

Any natural sample of chlorine contains 75% of ³⁵₁₇Cl and 25% ³⁷₁₇Cl, i.e., these isotopes are in the ratio of 3:1 respectively. Thus, average atomic mass of chlorine is calculated as

$\dfrac{(35\times3) + (37\times1)}{4}$ = 35.5. Therefore, atomic mass of chlorine atom is 35.5

Question 17

(a) What is meant by 'atomic number of an element'?

(b) Complete the table given below:

(c) Write down the electronic configuration of

(i) Chlorine atom

(ii) Chloride ion

Answer

(a) Atomic number of an element is the number of:

• electrons present in its neutral atom.
• protons present in the nucleus of its atom.
• positive charge in the nucleus of its atom.
• it is denoted by Z.

(b) Complete the table given below:

(c) Write down the electronic configuration of

(i) Chlorine atom — 2, 8, 7

(ii) Chloride ion — 2, 8, 8

Question 18

Draw diagrams representing the atomic structures of the following

(a) Sodium atom

(b) Chloride ion

(c) Carbon atom

(d) Oxide ion.

Answer

(a) Sodium atom —

(b) Chloride ion —

(c) Carbon atom —

(d) Oxide ion —

Question 19

Complete the following table relating to the atomic structure of some elements.

Answer

Question 20

Give reasons

(a) Physical properties of isotopes are different.

(b) Argon does not react.

(c) Actual atomic mass is greater than mass number.

(d) ³⁵₁₇Cl and ³⁷₁₇Cl do not differ in their chemical reactions.

Answer

(a) Physical properties depend on the atomic mass (mass number) and isotopes have different mass numbers due to different number of neutrons hence they differ in their physical properties.

(b) Argon [2, 8, 8] does not react or do not combine either with other atoms or among themselves to form molecules, since their octet is already complete, hence, they are in a stable state and their combining capacity is zero.

(c) Actual atomic mass is greater than the mass number because the mass number is a whole number approximation of atomic mass unit. Actual atomic mass is fractional as it is the weighted average of all the naturally occurring isotopes of that element.

(d) Chemical properties are determined by the electronic configuration of an atom and electronic configuration of both isotopes of chlorine is same, i.e., electronic configuration of ³⁵₁₇Cl is [2,8,7] and electronic configuration of ³⁷₁₇Cl is [2,8,7], hence they both have same chemical properties.

Question 21

Give the use of the following isotopes:

(a) Radioactive isotopes

(b) ²³⁵U

(c) ⁶⁰Co

(d) ¹³¹I

(e) ¹⁴C

Answer

(a) Radioactive isotopes — Used in industry to detect the leakage in underground oil pipelines, gas pipelines and water pipelines.

(b) ²³⁵U — Used as fuel in nuclear reactors.

(c) ⁶⁰Co — used in radiotherapy for treating cancer and other diseases.

(d) ¹³¹I — Used in treatment of goitre.

(e) ¹⁴C — Used for determining the age of historical and geological material.

Question 22

How does the Modern atomic theory contradict and correlate with Dalton's atomic theory?

Answer

Contradictions with Modern atomic theory:

Correlation with Modern atomic theory:

The modern atomic theory however correlates with Dalton's atomic theory in the fact that atoms are the smallest unit of matter taking part in a chemical reaction and in a given compound the relative number and kind of atoms is a constant.

Question 23

Explain:

(a) octet rule for formation of a chemical compound.

(b) duplet rule for formation of hydrogen.

Answer

(a) For an atom to achieve stable electronic configuration it must have 8 electrons in outermost orbit like that of noble gases other than Helium. This is known as Octet rule.

(b) For an atom of hydrogen to achieve stable electronic configuration it must have 2 electrons in the first shell [outermost] like that of noble gas Helium. So, two hydrogen atoms share one electron each to become stable. This is known as Duplet rule.

Question 24

An element

A has atomic number 7, mass number 14

B has electronic configuration 2, 8, 8

C has 13 electrons, 14 neutrons

D has 18 protons 22 neutrons

E has electronic configuration 2, 8, 8, 1

State

(i) valency of each element

(ii) which one is a metal

(iii) which is a non-metal

(iv) which is an inert gas.

Answer

(i) Valency of each element:

Element A : Atomic number (number of electrons) = 7

Electronic configuration = 2, 5

Valency of A = 8 - 5 = 3

Element B : Electronic configuration 2, 8, 8

Valency of B = Zero

Element C : has 13 electrons

Electronic configuration = 2, 8, 3

Valency of C = 3

Element D : Protons = Electrons = 18

Electronic configuration = 2, 8, 8

Valency of D = Zero

Element E : Electronic configuration = 2, 8, 8, 1

Valency of E = 1

(ii) C and E are metals.

(iii) A is a non-metal.

(iv) B and D are inert gases.

Question 25

Elements X, Y and Z have atomic numbers 6, 9 and 12 respectively. Which one :

(a) forms an anion

(b) forms a cation

(c) has four electrons in its valence shell?

Answer

(a) Y (2, 7) forms an anion.

(b) Z (2, 8, 2) forms a cation.

(c) X (2, 4) has four electrons in the valence shell.

Exercise 4(D)

Question 1

How do atoms attain noble gas configuration?

Answer

Atoms attain noble gas configuration either by gaining electrons or losing electrons or sharing of electrons with other atoms of the same element or another element to form a chemical compound.

Question 2

Define electrovalent bond.

Answer

The chemical bond formed between two atoms by transfer of one or more electrons from the atom of a metallic (electropositive) element to an atom of a non-metallic (electronegative) element is called ionic bond or electrovalent bond.

Question 3

Elements are classified as metals, non-metals, metalloids and inert gases. Which among them form electrovalent bonds?

Answer

Bonds formed between metals and non-metals are electrovalent bonds.

Metals have 1, 2 or 3 electrons in their valence shell. They have a tendency to lose their valence electrons. So they combine with non-metals which have 5, 6 or 7 electrons in their valence shell and thus have a tendency to gain electron(s) and form electrovalent bonds.

Question 4

(a) An atom X has three electrons more than the noble gas configuration. What type of ion will it form?

(b) Write the formula of its (X)

(i) sulphate

(ii) nitrate

(iii) phosphate

(iv) carbonate

(v) hydroxide.

Answer

(a) Since there are three electrons more than the noble gas configuration in the atom so the atom will try to lose three electron to gain noble gas configuration. Therefore, it will form a cation X³⁺.

Formula of its:

(i) Sulphate — X₂(SO₄)₃

(ii) Nitrate — X(NO₃)₃

(iii) Phosphate — XPO₄

(iv) Carbonate — X₂(CO₃)₃

(v) Hydroxide — X(OH)₃

Question 5

Mention the basic tendency of an atom which causes it to combine with other atoms.

Answer

The basic tendency of an atom which makes it to combine with other atoms is the tendency of elements to acquire the nearest noble gas configuration in their outermost orbit and become stable

Question 6

What type of compounds are usually formed between metals and non-metals and why?

Answer

Compounds formed between metals and non-metals are electrovalent compounds.

The metals have 1, 2 or 3 electrons in their valence shell. They have a tendency to lose their valence electrons. So they combine with non-metals which have five, six or seven electrons in their valence shell and thus have a tendency to gain electron(s) and form electrovalent compounds.

Question 7

(a) In the formation of the compound XY₂, an atom X gives one electron to each Y atom. What is the nature of bond in XY₂?

(b) Draw the orbit structure of this compound (XY₂).

Answer

(a) Nature of bond is ionic because the atom X gives one electron to each Y atom.

(b) Orbit structure of XY₂ :

Question 8

An atom X has electronic configuration 2,8,7. It combines with Y having 1 electron in its outermost shell.

(a) What type of bond will be formed between X and Y?

(b) Write the formula of the compound formed.

Answer

(a) Ionic Bond
Reason — Since X has 7 electrons in its outermost shell and Y has only one electron in its outermost shell. So, Y loses its one electron and X gains that electron to form an ionic bond.

(b) The formula of the compound is XY.

Question 9

Draw orbit structure diagram of

(a) sodium chloride (NaCl)

(b) calcium oxide (CaO)

[Atomic numbers Na = 11, Ca = 20, Cl = 17, O = 8]

Answer

(a) Orbit structure of NaCl is shown below:

(b) Orbit structure diagram of calcium oxide is shown below:

Question 10

Compare :

(a) sodium atom and sodium ion

(b) chlorine atom and chloride ion, with respect to

(i) atomic structure, (ii) electrical state

Answer

(a) Comparison between sodium atom and sodium ion

(b) Comparison between chlorine atom and chloride ion

Question 11

The electronic configuration of fluoride ion is the same as that of a neon atom. What is the difference between the two ?

Answer

The electronic configuration of Fluoride ion is the same as that of a neon atom i.e., both have 8 electrons in their valence shell but Fluoride attain that configuration by gaining an electron so it is negatively charged but neon atom have its own 8 valence electrons so it is neutral i.e., it has no charge on it.

Question 12

(a) What do you understand by redox reactions?

(b) Explain oxidation and reduction in terms of loss or gain of electrons.

Answer

(a) Redox Reaction — A reaction in which oxidation and reduction occur simultaneously is called an oxidation-reduction reaction or redox reaction.

In redox reaction there is a transfer of electrons between two atoms.

(b) The reaction in which electron(s) is gained is called a reduction reaction and the reaction in which there is a loss of electron(s) is called an oxidation reaction

Question 13

Potassium (At No. 19) and chlorine (At No. 17) form a compound. Explain the formation of the compound on the basis of:

(a) oxidation

(b) reduction

(c) oxidising agent

(d) reducing agent.

Answer

The electronic configuration of:

Potassium (19) = 2, 8, 8, 1

Chlorine (17) = 2, 8, 7

The reaction is :

2K + Cl₂ ⟶ K⁺ + 2Cl^-

(i) Oxidation : 2K ⟶ K⁺
    Potassium undergoes oxidation reaction as it loses an electron.

(ii) Reduction : Cl₂ ⟶ 2Cl^-
     Chlorine undergoes reduction reaction as it gains an electron.

(iii) Chlorine acts as an oxidising agent and gets reduced.

(iv) Potassium acts as a reducing agent and gets oxidised.

Exercise 4(E) — Multiple Choice Type

Question 1

(Choose the correct answer from the options given below).

The bond formed between a metal and non-metal is called:

1. Polar covalent
2. Electrovalent
3. Covalent
4. Coordinate

Answer

Electrovalent

Reason — Metals become positively charged when they give away electrons and non-metals become negatively charged when they accept electrons. Due to electrostatic attraction between the two oppositely charged ions electrovalent (ionic) bonds are formed.

Question 2

(Choose the correct answer from the options given below).

The most ionic compound is:

1. NaCl
2. KBr
3. CsF
4. MgCl₂

Answer

CsF

Reason — Caesium fluoride CsF, is the most ionic as caesium has the maximum tendency to loose an electron while fluorine has the maximum tendency to gain an electron.

Question 3

(Choose the correct answer from the options given below).

The electronic configuration of Ca²⁺:

1. 2, 8, 8, 2
2. 2, 8, 8
3. 2, 8, 8, 4
4. 2, 8, 2

Answer

2, 8, 8

Reason — The number of electrons in calcium atom is 20 and the electronic configuration of a neutral calcium atom is 2, 8, 8, 2. Ca²⁺ has two electrons less so the electronic configuration becomes 2, 8, 8.

Question 4

(Choose the correct answer from the options given below).

The formation of electrovalent bond is an example of:

1. Oxidation reaction
2. Reduction reaction
3. Displacement reaction
4. Redox reaction

Answer

Redox reaction

Reason — In the formation of an electrovalent bond, the transfer of electron(s) is involved. The electropositive atom loses electrons and undergoes oxidation, while the electronegative atom gains electrons and undergoes reduction. This is known as redox reaction.

Question 5

(Choose the correct answer from the options given below).

Ionic compounds are generally:

1. Liquids
2. Gases
3. Liquids of very low boiling points
4. Solids

Answer

Solids

Reason — Ionic compounds have strong electrostatic forces of attraction between their ions, which cannot be separated easily. Thus, they are hard, crystalline solids consisting of ions.

Question 6

(Choose the correct answer from the options given below).

An example of a double covalent bond is:

1. Nitrogen molecule
2. Hydrogen molecule
3. Oxygen molecule
4. Chlorine molecule

Answer

Oxygen molecule

Reason — To attain the stable electronic configuration of the nearest noble gas neon, oxygen needs two electrons.

When two oxygen atoms come closer, each contributes two electrons and so they have two shared pair of electrons between them. Both atom attain an octet, resulting in the formation of a double covalent bond [O=O] between them.

Question 7

(Choose the correct answer from the options given below).

Which of the following is not a non polar covalent molecule:

1. Chlorine
2. Methane
3. Hydrogen chloride
4. Carbon tetrachloride

Answer

Hydrogen chloride

Reason — The covalent bond is said to be polar when the shared pair of electrons are not at equal distance between the two atoms. This results in the development of fractional positive charge and negative charge on them and they ionise in water. For example: hydrogen chloride.

Question 8

(Choose the correct answer from the options given below).

The total number of atoms present in a carbon tetrachloride molecule is:

1. 4
2. 3
3. 5
4. 6

Answer

5

Reason — One carbon atom and 4 chlorine atoms combine to form a carbon tetrachloride (CCl₄) molecule.

Question 9

(Choose the correct answer from the options given below).

Three pair of electrons are shared in a molecule of:

1. Oxygen
2. Hydrogen
3. Methane
4. Nitrogen

Answer

Nitrogen

Reason — To attain the stable electronic configuration of the nearest noble gas neon, nitrogen needs three electrons.

When two nitrogen atoms come closer, each contributes three electrons and so they have three shared pair of electrons between them. Both atom attain an octet, resulting in the formation of a triple covalent bond [N≡N] between them.

Question 10

(Choose the correct answer from the options given below).

In the formation of CCl₄ molecule, the nearest noble gas configuration attained by carbon and chlorine respectively are :

1. Argon and neon
2. Helium and neon
3. Neon and argon
4. Helium and argon

Answer

Neon and argon

Reason —

Question 11

(Choose the correct answer from the options given below).

The characteristic of an electrovalent compound is that :

1. It is formed by sharing of electrons
2. It is formed between metals and non-metals
3. It is formed between two non-metals
4. It often exists as a liquid.

Answer

It is formed between metals and non-metals

Reason — Bonds formed between metals and non-metals are electrovalent bonds.

Metals have 1, 2 or 3 electrons in their valence shell. They have a tendency to lose their valence electrons. So they combine with non-metals which have 5, 6 or 7 electrons in their valence shell and thus have a tendency to gain electron(s) and form electrovalent bonds.

Question 12

(Choose the correct answer from the options given below).

When a metal atom becomes an ion:

1. It loses electrons and is oxidized.
2. It gains electrons and is reduced.
3. It gains electrons and is oxidized
4. It loses electrons and is reduced.

Answer

It loses electrons and is oxidized.

Reason — As oxidation is loss of electrons so the metal atom is oxidised after losing electrons.

Question 13

(Choose the correct answer from the options given below).

An element Y has 2 electrons in the 2nd orbit and X has 7 electrons in its 3rd orbit. The compound formed between them has the formula:

1. XY₂
2. YX₂
3. YX
4. Y₇X₂

Answer

XY₂

Reason — As element X has 2 electrons in its 2nd orbit, so the electronic configuration of X is [2, 2] and Y has 7 electrons in the third orbit so its electronic configuration as [2, 8, 7]

X donates it two valence electrons and completes its octet and two atoms of Y accept the electrons to attain the octet, hence the compound formed is XY₂.

Exercise 4(E) — Very Short Answer Type

Question 1

Match the atomic numbers 4, 8, 10, 15 and 19 with each of the following:

(a) Element which can form trivalent ion.

(b) An element with 4 shells.

(c) Element with 6 valence electrons.

(d) Element which does not form ion.

Answer

Explanation:

1. Electronic configuration of an element with atomic number 15 is [2, 8, 5], hence, it needs three electrons to complete its octet and forms a trivalent ion.
2. Electronic configuration of an element with atomic number 19 is [2, 8, 8, 1], hence, it has K, L, M, N shells.
3. Electronic configuration of an element with atomic number 8 is [2, 6], hence it has 6 electrons in the outermost shell called valence electrons.
4. Electronic configuration of an element with atomic number 10 is [2, 8], so its octet is complete and it does not form ions.

Question 2

Identify the following reactions as either oxidation or reduction:

(i) O + 2e^- ⟶ O^2-

(ii) K - e^- ⟶ K⁺

(iii) Fe³⁺ + e^- ⟶ Fe²⁺

(iv) Zn ⟶ Zn²⁺ + 2e^-

Answer

(i) Reduction, since there is a gain of 2 electrons by Oxygen.

(ii) Oxidation, since there is a loss of an electron by Potassium.

(iii) Reduction, since there is a gain of an electron by Iron.

(iv) Oxidation, since there is a loss of two electrons by Zinc.

Question 3

Give the terms for the following definitions:

(a) A bond formed by a shared pair of electrons, each bonding atom contributing one electron to the pair.

(b) A bond formed by transfer of electron(s).

Answer

(a) Single Covalent bond

(b) Electrovalent bond

Question 4

Name or state the following.

(a) An element having valency zero.

(b) Metal with valency one.

(c) Atoms of the same element differing in mass numbers.

(d) Elements having same mass number but different atomic number.

(e) Bond formed by transfer of electrons(s)

(f) Ion formed by gain of electron(s)

Answer

(a) Helium (Inert element)

(b) Lithium

(c) Isotopes

(d) Isobars

(e) Electrovalent or Ionic bond

(f) Anion

Exercise 4(E) — Short Answer Type

Question 1

(a) Define covalent (molecular) bond.

(b) Give an example of covalent bond formed by

(i) similar atoms

(b) dissimilar atoms

Answer

(a) Covalent Bond — The chemical bond that is formed between two combining atoms by mutual sharing of one or more pair of electrons is called a covalent bond. Depending on the number of electron pairs shared : bond is single [-], double [=], or triple [≡] covalent.

(b) (i) Bond formed between two H atoms; H-H

A hydrogen atom has one electron in its only shell. It needs one more electron to attain duplet. To meet this need, hydrogen atom shares its electron with another hydrogen atom. Thus, one electron each is contributed by the two atoms of hydrogen, and the resulting pair of electrons is mutually shared by both the atoms to form a hydrogen molecule.

(ii) Bond formed between a hydrogen atom and chlorine atom; H‒Cl

In the case of hydrogen chloride molecule, hydrogen atom shares an electron pair with chlorine atom, such that hydrogen acquires a duplet configuration and chlorine an octet, resulting in the formation of a single covalent bond.

Question 2

An element A has 1 electron in its first shell. It combines with element B having 7 electrons in its third shell. What type of bond is formed ?

Answer

Element A with 1 electron in its first shell is hydrogen, and element B with 7 electrons in its third shell is chlorine. So, a single covalent bond is formed between hydrogen and chlorine by sharing one pair of electron.

Question 3

If electrons are getting added to an element Y; then

(a) what charge will Y have?

(b) is Y getting oxidized or reduced ?

Answer

(i) Y will have negative charge as electrons are getting added to it.

(ii) Y is getting reduced as reduction is gain of electrons.

Question 4

(a) Elements X, Y and Z have atomic numbers 6, 9 and 12 respectively. Which one:

(i) forms an anion,

(ii) forms a cation,

(b) State the type of bond between Y and Z and give, its molecular formula.

Answer

(a) X[6] = Electronic configuration [2,4]

Y[9] = Electronic configuration [2,7]

Z[12] = Electronic configuration [2,8,2]

(i) Y gains one electron to attain a stable octet, hence becomes negatively charged anion.

(ii) Z loses two electrons to attain a stable octet hence becomes positively charged cation.

(b) Ionic bond with molecular formula ZY₂.

Question 5

Potassium Chloride is an electrovalent compound, while hydrogen chloride is a covalent compound. But, both conduct electricity in their aqueous solutions. Why?

Answer

Potassium chloride being an electrovalent compound consists of ions. In aqueous solution state, the electrostatic forces between them weaken and the ions become mobile conducting electricity.

On the other hand, hydrogen chloride is a polar covalent compound. The shared electron pair is shifted towards Chlorine thereby developing a slight negative charge on it and a slight positive charge on hydrogen atom. In its aqueous solution state, hydrogen chloride ionises into Hydronium and Chloride ions as shown in the equation below. These ions help in conducting electricity.

HCl + H₂O ⇌ H₃O⁺ + Cl^-

Question 6

Name two compounds that are covalent when taken pure but produce ions when dissolved in water.

Answer

Hydrogen Chloride (HCl) & Ammonia (NH₃)

Question 7

An element M burns in oxygen to form an ionic compound MO. Write the formula of the compounds formed if this element is made to combine with chlorine and sulphur separately.

Answer

With Chlorine — MCl₂

With Sulphur — MS

Explanation:

Given, M reacts with oxygen to form an ionic bond MO. Oxygen needs 2 electrons to complete its octet forming the anion O^2-. As the compound formed is MO, it means that M has 2 electrons in its valence shell which it donates to Oxygen to attain a stable electronic configuration forming a cation M²⁺.

Each Chlorine atom accepts 1 electron to complete its octet so 2 Chlorine atoms combine with 1 atom of M to form MCl₂. Each Sulphur atom accepts 2 electrons to complete its Octet so 1 Sulphur atom combines with 1 atom of M to form MS.

Question 8

State the type of bonding in the following molecules.

(a) water,

(b) calcium oxide,

(c) hydrogen chloride

Answer

Covalent bonding
Reason — The valency of hydrogen element is 1 and that of oxygen is 2. Hydrogen needs one electrons to attain stable duplet structure of nearest noble gas - He [2] and oxygen needs two electrons to attain stable octet structure of nearest noble gas - Ne [2,8]
Each of the two hydrogen atoms shares an electron pair with the oxygen atom such that hydrogen acquires a duplet configuration and oxygen an octet configuration resulting in the formation of two single covalent bonds [H-O-H] in the molecule of water.

(ii) Electrovalent bonding
Reason — The valency of calcium (2,8,8,2) element and that of oxygen (2,6) is 2. Calcium donates two electrons and oxygen takes up those two electrons and electrovalent bond is formed.

(iii) Polar covalent bonding Reason — In hydrogen chloride, the strong nuclear charge of the chlorine atom attracts the shared electron pair towards itself, i.e., negative charge shifts towards the chlorine atom thereby developing a slight negative charge on it. The hydrogen atom develops a slight positive charge. Therefore, a polar covalent bond is formed as shown below :

Question 9

The atomic species X and Y have different number of protons but the same number of nucleons. While atoms A and B have the same number of protons but different number of nucleons.

(a) Which of them are

(i) isotopes

(ii) isobars ?

(b) Give one example of each.

Answer

(a) (i) A and B are isotopes, as they have same atomic number but different mass number.

(ii) X and Y are isobars, as they have same mass number but different atomic number.

(b) (i) Examples of isotopes — ³⁵₁₇Cl, ³⁷₁₇Cl

(ii) Examples of isobars — ⁴⁰₁₈Ar, ⁴⁰₂₀Ca

Exercise 4(E) — Descriptive Type

Question 1

Covalent bonds can be single, double or triple covalent bond.

(a) How many electrons are shared in each?

(b) Give an example of each type.

Answer

A single covalent bond is formed by sharing of one pair of electrons between the atoms, each atom contributing one electron.

Ex: Molecule of hydrogen [H-H]

A double covalent bond is formed by sharing of two pairs of electrons between two atoms.

Ex: Molecule of oxygen [O=O]

A triple covalent bond is formed by sharing of three pairs of electrons between two atoms.

Ex: Molecule of nitrogen [N≡N]

Question 2

Taking MgCl₂, as an example of electrovalent bond, CCl₄ as a covalent bond. Give a difference between electrovalent and covalent bonds.

Answer

Magnesium loses its two valence electrons to acquire a stable octet. The two chlorine atoms accept one each of these electrons to acquire a stable octet. Thus, electrovalent bond in MgCl₂ is formed by transfer of electrons.
When a molecule of CCl₄ is to be formed, one atom of carbon shares four electron pairs, one with each of the four atoms of chlorine. Thus, covalent bond is formed by sharing of electrons.

Question 3

Give orbital diagram of the following :

(a) magnesium chloride,

(b) nitrogen

(c) methane

(d) hydrogen chloride

Answer

(a) Orbital diagram of Magnesium Chloride

(b) Orbital diagram of Nitrogen

(c) Orbital diagram of Methane

(d) Orbital diagram of Hydrogen Chloride

Question 4

Metal M forms a chloride with the formula MCl₂. What type of bond is in MCl₂? Write the formula of the compound when M combines with sulphur, oxygen and nitrogen.

Answer

The bond formed between metal (M) and non-metal (chlorine) is ionic bond.

As M bonds with two chlorine atoms having electronic configuration [2,8,7], so we can say that M donates two electrons to two chlorine atoms to complete their octet and therefore, it has a valency of 2.

When metal (M) having +2 valency combines with sulphur [2,8,6] having -2 valency, M donates its two valence electrons to sulphur and formula of compound formed is MgS

When metal(M) having +2 valency combines with oxygen [2,6] having -2 valency, M donates its two valence electrons to oxygen and formula of compound formed is MgO

When metal (M) atoms having +2 valency combines with nitrogen [2,5] having -3 valency then an ionic bond is formed by the transfer of 6 electrons from 3 metal (M) atoms to 2 nitrogen atoms, hence the formula of the compound is Mg₃N₂.

Question 5

Explain the following :

(a) Mass of an atom is concentrated inside the nucleus of the atom.

(b) Atoms combine by transfer and sharing of electron(s).

(c) An element has atoms with different mass number.

(d) Carbon-12 and carbon-14 both show similar chemical properties

Answer

(a) Atom is made of primarily three sub-atomic particles namely electrons, protons and neutrons. An electron has negligible mass, and protons and neutron lie inside the nucleus, hence, mass of an atom is majorly present in the nucleus.

(b) Elements tend to combine with one another to attain the stable electronic configuration of the nearest noble gas. This is achieved either by gaining electrons or losing electrons or sharing of electrons with other atoms of the same element or another element to form a chemical compound.

(c) Isotopes are the elements having the same atomic number but different mass numbers.

For example : ²⁴₁₂Mg and ²⁶₁₂Mg

Isotopes differ in the number of neutrons.

(d) Chemical properties are determined by the electronic configuration of an atom and electronic configuration of both isotopes of carbon is same, i.e., electronic configuration of C-12 is [2,4] and electronic configuration of C-14 is [2,4], hence they both have same chemical properties.

Question 6

With the help of a diagram, state the number of bonds and the type of bonds in:

(a) ethene molecule

(b) ethyne molecule

Answer

(a) Ethene molecule [C₂H₄] — It has one double covalent bond between the 2 Carbon atoms and four single covalent bonds with hydrogen atoms.

(b) Ethyne molecule [C₂H₂] — It has one triple covalent bond between the 2 Carbon atoms and 2 single covalent bonds with hydrogen atoms.

Exercise 4(E) — Structured/Application/Skill Type

Question 1

The atom of an element A has 6 electrons in its M shell.

(a) Write the electronic configuration of element A.

(b) What is the atomic number of element A?

(c) Is it a metal or a non-metal?

(d) What type of ion will be formed by an atom of element A? Write the symbol of the ion formed.

(e) What could element A be ?

(f) Draw the orbital structure of A and its ion.

Answer

(a) [2, 8, 6], as it has three shells — K, L, M.

(b) 16, as atomic number is the total number of electrons so, 2+8+6 = 16

(c) Non-metal, as it has 6 electrons in the valence shell and will accept 2 electrons to complete its octet.

(d) Anion, A^2-, as element A has 6 valence electrons so it has a valency of -2 and it will take two electrons, complete its octet and become a negatively charged ion.

(e) Sulphur, as sulphur also has the electronic configuration of [2, 8, 6].

(f) Orbital structure of A and its ion is shown below:

Question 2

An atom of an element Y may be written as ⁹₄Y.

(a) What does the figure 9 indicate?

(b) What does the figure 4 indicate?

(c) What is the number of protons in atom Y?

(d) What is the number of neutrons in atom Y?

(e) What is the number of electrons in atom Y?

(f) How many electrons are there in the outermost shell of an atom of element Y?

(g) Write the symbol of ion formed by an atom of element Y?

Answer

(a) Mass number is indicated by figure 9, which is the sum of protons and neutrons in the nucleus of an atom.

(b) Atomic number is indicated by figure 4, which is the number of electrons present in a neutral atom.

(c) 4, as the atomic number also indicates the number of protons in the nucleus of an atom.

(d) 5, number of neutron = mass number - number of protons = 9-4=5

(e) 4, as the atomic number is 4, which is the number of electrons present in a neutral atom.

(f) 2, its electronic configuration is [2, 2]

(g) Y²⁺, as it has the valency of +2, hence it loses two valence electrons and becomes a positively charged ion.

Question 3

Atom A is represented as ²⁰⁹₈₂A, atom B as ²⁰⁹₈₃B and atom C as ²¹¹₈₂C

(a) How many electrons, protons and neutrons do A, B and C have?

(b) Which two atoms have the different number of nucleons?

(c) Name the term which can be used for A and C atoms.

(d) Define the term mentioned in part (c) above.

Answer

(a)

(b) A and C have different number of nucleon, because the number of protons is same in both but number of neutrons are different.

(c) Isotopes

(d) Isotopes are atoms of the same element having the same atomic number but different mass numbers.

Question 4

(a) Name the charged particles which attract one another to form electrovalent compounds.

(b) In the formation of electrovalent compounds; electrons are transferred from one element to another. How are electrons involved in the formation of a covalent compound ?

(c) The electronic configuration of nitrogen is (2, 5). How many electrons in the outer shell of a nitrogen atom are not involved in the formation of a nitrogen molecule ?

(d) In the formation of magnesium chloride (by direct combination between magnesium and chlorine), name the substance that is oxidized and the substance that is reduced.

Answer

(a) Cation [positively charged] and anion [negatively charged]

(b) In the formation of covalent compounds, the electrons are shared between the combining atoms. Depending on the number of electron pairs shared : bond is single [-], double [=], or triple [≡] covalent.

(c) Two electrons in the outer shell of a nitrogen atom are not involved in the formation of a nitrogen molecule.
Reason — Nitrogen has 5 electrons in the valence shell whereas it forms triple bond to fulfill it's octet and hence, 2 electrons are left in the outer shell of a nitrogen atom which are not involved in the formation of a nitrogen molecule.

(d) Magnesium is oxidized and Chlorine is reduced.
Reason — In the formation of magnesium chloride the reaction is :

Mg + Cl₂ ⟶ Mg²⁺ + 2Cl^-

Oxidation : Mg ⟶ Mg²⁺

Reduction : Cl₂ ⟶ 2Cl^-

Question 5

An element X has 2 electrons in its M shell, it forms bond with an element Y which has 7 electrons in its third orbit.

(a) Write the formula of the compound formed.

(b) Which nearest inert gas electronic configuration will element X and Y acquire?

(c) Show using an orbital diagram the formation of compound between X and Y.

Answer

(a) XY₂
Reason — As element X has 2 electrons in its M shell, so the electronic configuration of X is [2, 8, 2] and Y has 7 electrons in the third shell so its electronic configuration is [2, 8, 7]
X donates it two valence electrons and completes its octet and two atom of Y accepts the electrons to attain the octet, hence the compound formed is XY₂.

(b) The nearest inert gas electronic configuration for element X is 2, 8 i.e., Neon while that for element Y is 2, 8, 8 i.e, Argon.

(c) Orbital diagram of XY₂

Question 6

In the formation of (i) Oxygen molecule (ii) Carbon tetrachloride molecule, state the following:

(a) Electronic configuration of nearest inert gas attained.

(b) How many electrons are shared/transferred in bond formation?

(c) Which type of bonds these compounds form?

(d) Draw the orbital diagrams?

Answer

(i) In the formation of Oxygen molecule:

(a) Neon (10) 2,8, because electronic configuration of oxygen is [2,6] so to attain the stable configuration of nearest noble gas Neon, oxygen needs two electrons.

(b) Two pairs of electrons are shared, and both oxygen atoms attain their octet.

(c) Double Covalent bond, as the bond between two oxygen atoms is formed by sharing of electrons.

(d) Orbital Diagram of the formation of Oxygen molecule:

(ii) In the formation of Carbon tetra chloride molecule:

(b) Four pair of electrons are shared, as carbon needs four electrons and chlorine needs one electron to complete their octet, hence four chlorine atoms share their one electron each with carbon and all the atoms attain a stable configuration.

(c) Single Covalent bond, as the bond between carbon and 4 chlorine atoms is formed by sharing one pair of electrons.

(d) Orbital Diagram of the formation of Carbon tetrachloride molecule:

Exercise 4(E) — Numericals

Question 1

What is the average atomic mass of bromine if it has 49.7% ⁷⁹₃₅Br and 50.3% ⁸¹₃₅Br?

Answer

Given,

49.7% of ⁷⁹₃₅Br and

50.3% of ⁸¹₃₅Br

These isotopes are in the ratio = $\dfrac{49.7}{50.3}$ = $\dfrac{497}{503}$

Average atomic mass = $\dfrac{(79 \times 497)+(81 \times 503)}{1000}$

$= \dfrac{39263+(40743)}{1000} \\[1em] = \dfrac{80006}{1000} \\[1em] = 80.006 \text{ amu} \\[1em] \approx 80 \text{ amu}$

∴ Average atomic mass of bromine = 80 amu

Question 2

A sample of an element A contains two isotopes ¹⁶₈A and ¹⁸₈A. If the average atomic mass of the element is 16.2 amu, calculate the percentage of the two isotopes in this sample.

Answer

Let,

the percentage of ¹⁶₈A be X and

percentage of ¹⁸₈A be 100-X

Average atomic mass = $\dfrac{(16 \times \text{X})+[18 \times \text{(100-X)}]}{100}$ = 16.2

⇒ 16X + 1800 - 18X = 1620

⇒ -2X + 1800 = 1620

⇒ -2X = 1620 - 1800

⇒ 2X = 180

⇒ X = $\dfrac{180}{2}$ = 90

∴ 100 - X = 100 - 90 = 10

Hence, percentage of the two isotopes in this sample are 90% of ¹⁶₈A and 10% of ¹⁸₈A