Atomic Structure

The smallest particle of matter exhibiting all its properties is :

1. a radical
2. an atom
3. a molecule
4. an electron

Answer

an atom

Reason — The atom is the smallest particle that exhibits all the properties of a particular element.
A radical is an atom or group of atoms with a charge. An electron is a subatomic particle. A molecule is the small particle of a substance (element or compound) that can exist independently.

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

1. electron
2. proton
3. neutron
4. nucleus

Answer

nucleus

Reason — Rutherford's alpha-particle scattering experiment showed that an atom has a small, centrally located, positively charged nucleus. A few alpha particles were strongly deflected or bounced back due to repulsion by this positively charged nucleus.
The electron was discovered by J. J. Thomson. The neutron was discovered by James Chadwick.

The outermost shell of an atom is known as:

1. valency
2. valence electrons
3. nucleus
4. valence shell

Answer

valence shell

Reason — The outermost shell of an atom is known as its valence shell.
Valency is the combining capacity of an atom, not a shell. These are the electrons present in the outermost shell, not the shell itself. The nucleus is the central part of the atom, not the outermost shell.

The number of valence electrons present in magnesium is :

1. two
2. three
3. four
4. five

Answer

two

Reason — Magnesium (Mg) has an atomic number of 12, meaning it has 12 electrons. The electron configuration of magnesium is: 2, 8, 2.

The subatomic particle with positive charge is:

1. proton
2. neutron
3. electron
4. nucleon

Answer

proton

Reason — The subatomic particle with positive charge is the proton, which is located inside the nucleus.
A neutron has no charge. An electron carries a negative charge. Nucleon is a collective term for protons and neutrons.

If the atomic number of an atom is 17 and mass number is 35, then number of neutrons will be:

1. 35
2. 17
3. 18
4. 52

Answer

18

Reason — If the atomic number of an atom is 17 and mass number is 35, then number of neutrons will be 18.
Neutrons = Mass Number − Atomic Number

The number of electrons in an atom is equal to the number of :

1. protons in a neutral atom
2. neutrons in a neutral atom
3. nucleons in a neutral atom
4. none of the above

Answer

protons in a neutral atom

Reason — In a neutral atom, the number of electrons is always equal to the number of protons, so their positive and negative charges cancel out, resulting in no overall charge.
The number of neutrons can vary (isotopes), so it is not always equal to electrons. Nucleons = protons + neutrons, so this number is greater than electrons.

The sum of number of protons and number of neutrons present in the nucleus of an atom is called its :

1. mass number
2. atomic number
3. valency
4. none of these

Answer

mass number

Reason — The sum of number of protons and number of neutrons present in the nucleus of an atom is called its mass number.
Atomic number is the number of protons only. Valency is the combining capacity of an atom.

The correct electronic configuration of sodium is:

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

Answer

2, 8, 1

Reason — The correct electronic configuration of sodium is 2, 8, 1.

Assertion (A): Matter consists of very small particles called atoms.

Reason (R): Atoms are indivisible.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

A is true but R is false.

Explanation — A is true as matter consists of very small particles called atoms.
R is false as atoms are not indivisible, they can be further divided into fundamental particles electrons, protons and neutrons.

Therefore, A is true but R is false is the correct option.

Assertion (A): An atom is electrically neutral.

Reason (R): The number of protons and the number of electrons are equal in an atom.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

Both A and R are true and R is the correct explanation of A.

Explanation — A is true as atoms have no overall charge.
R is true since the number of protons and the number of electrons are equal in an atom, making the atom electrically neutral.

Therefore, Both A and R are true and R is the correct explanation of A is the correct option.

Assertion (A): The nucleus of an atom is a centrally located mass containing protons only.

Reason (R): The nucleus of an atom is positively charged.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

A is false but R is true.

Explanation — A is false as the nucleus of an atom is a centrally located mass containing both protons and neutrons.
R is true as neutrons have no charge and protons are positively charged; therefore, the nucleus of an atom is positively charged.

Therefore, A is false but R is true is the correct option.

Assertion (A): The sum of the number of protons and neutrons present in the nucleus of an atom of an element is called its mass number.

Reason (R): The number of protons in the nucleus of an atom of an element is called its atomic number.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

Both A and R are true but R is not the correct explanation of A.

Explanation

• Assertion (A) is correct because
  Mass number = number of protons + number of neutrons in the nucleus.

• Reason (R) is also correct because
  Atomic number = number of protons in the nucleus.

However, knowing what atomic number is does not explain what mass number is. Mass number depends on both protons and neutrons, while the Reason only talks about protons.

Therefore, Both A and R are true but R is not the correct explanation of A is the correct option.

Assertion (A): The circular orbits surrounding the nucleus of an atom are also called as energy levels or shells.

Reason (R): These circular orbits are associated with a fixed amount of energy.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

Both A and R are true and R is the correct explanation of A.

Explanation — A is true as the circular orbits are also known as energy levels or shells.
R is true as each orbit is associated with a fixed amount of energy.

Therefore, Both A and R are true and R is the correct explanation of A is the correct option.

Fill in the blanks:

(a) Dalton said that ............... could not be divided.

(b) The fundamental particle of an atom carrying a negative charge is called ............... .

(c) An ion which has a positive charge is called a ............... .

(d) The outermost shell of an atom is known as ............... .

(e) The ............... of an atom is very hard and dense.

(f) Neutrons are ............... particles having mass equal to that of protons.

(g) Isotopes are the atoms of ............... element having the ............... atomic number but ............... mass number.

Answer

(a) atoms

(b) electron

(c) cation

(d) valence shell

(e) nucleus

(f) neutral

(g) same, same, different

Write True or False for the following statements:

(a) An atom on the whole has a positive charge.

(b) The maximum number of electrons in the first shell can be 8.

(c) The central part of an atom is called nucleus.

(d) The cathode rays in the discharge tube consist of protons.

(e) The neutrons are neutral and have a mass equal to 1 amu.

Answer

(a) False
Correct statement — An atom on the whole is electrically neutral.

(b) False
Correct statement — The maximum number of electrons in the first shell can be 2.

(c) True

(d) False
Correct statement — The cathode rays in the discharge tube consist of electrons.

(e) True

Match the following :

Answer

Name the following:

(a) The sub-atomic particle with negative charge and negligible mass.

(b) Protons and neutrons present in the nucleus.

(c) The electrons present in the outermost shell.

(d) Arrangement of electrons in the shells of an atom.

(e) The number of protons present in the nucleus of an atom.

(f) The sum of the number of protons and neutrons of an atom.

(g) Atoms of same element with same atomic number but a different mass number.

(h) The smallest unit of an element which takes part in a chemical reaction.

(i) The particle which is not present in an atom of hydrogen.

Answer

(a) Electrons

(b) Nucleons

(c) Valence electrons

(d) Electronic configuration

(e) Atomic number

(f) Mass number

(g) Isotopes

(h) Atoms

(i) Neutron

Define the following terms:

(a) Atomic number

(b) Mass number

(c) Nucleons

(d) Valence shell

Answer

(a) The number of protons present in the nucleus of the atom of an element is called its atomic number.

(b) The sum of the number of protons and the number of neutrons present in the nucleus of the atom of an element is called the mass number of that element.

(c) The protons and neutrons are collectively called as nucleons.

(d) The outermost shell of an atom is known as its valence shell or valence orbit.

Name three fundamental particles of an atom. Give their symbols with charge on each particle.

Answer

The three fundamental particles of an atom are:

1. Electrons — An electron is a negatively charged particle denoted by the symbol ⁰_-1e.
2. Protons — A proton is a positively charged particle denoted by the symbol ¹₊₁p.
3. Neutrons — A neutron is a neutral particle denoted by the symbol ¹₀n.

Which fundamental particle is present in anode rays ? Why anode rays are also called as canal rays.

Answer

The fundamental particle present in anode rays is the proton. Anode rays are also called canal rays because, during the experiment by E. Goldstein, these rays passed through the small holes (canals) in the perforated cathode, creating glowing channels.

What are cathode rays ? Which fundamental particle is present in these rays ?

Answer

William Crookes observed that when an electric current of high voltage was passed through the discharge tube, it started glowing due to the emission of rays from the cathode. He called these rays 'cathode rays'. Cathode rays contained negatively charged particles called electrons.

The atom of an element is made up of 4 protons, 5 neutrons and 4 electrons. What is its atomic number and mass number ? Represent the atom with a symbol.

Answer

Atomic number = number of protons = 4.

Mass number = number of protons + number of neutrons = 4 + 5 = 9.

The given atom is beryllium (Be).

Symbolic representation = ⁹₄Be.

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

Answer

Atomic number of sodium is 11. This means that a sodium atom has 11 protons in its nucleus and 11 electrons in its neutral atom.

Mass number of sodium is 23. This means that the total number of protons and neutrons is 23.

Number of neutrons = 23 - 11 = 12.

Differentiate between protons and neutrons.

Answer

Differences between protons and neutrons:

Differentiate between nucleons and nucleus

Answer

Differences between nucleons and nucleus:

Differentiate between atomic number and mass number

Answer

Differences between atomic number and mass number:

Why is Thomson's Atomic model called as Plum Pudding model ?

Answer

According to this model, an atom is a positively charged sphere in which electrons are embedded just like plums or dry fruits are distributed in a pudding. Therefore, it is known as the Plum Pudding Model.

Mention briefly the salient features of Dalton's atomic theory (five points).

Answer

The salient features 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. Atoms of an element combine in small numbers to form molecules of that element.
4. Atoms of an element combine with the atoms of another element in a simple whole number ratio to form molecules of a compound.
5. Atoms are the smallest unit of matter that take part in chemical reactions during which only rearrangement of atoms takes place.

What are the observations of the alpha particle scattering experiment done by Rutherford in order to determine the structure of an atom ?

Answer

In his experiment Rutherford bombarded a thin sheet of gold (of 0.00004 cm thickness) with fast-moving alpha particles in an evacuated chamber.

His observations were:

1. Most of the alpha particles passed straight through the foil without any deflection from their path.
2. A small fraction of them were deflected from their original path by small angles.
3. Only a few particles bounced back.

(a) What are the two main features of Rutherford's atomic model ?

(b) State its one drawback.

Answer

(a) The two main features of Rutherford's atomic model are:

1. An atom consists of a centrally located, positively charged nucleus. The entire mass of the atom is concentrated in the nucleus, which is very small and dense.
2. Electrons revolve around the nucleus in circular orbits called shells. An atom is electrically neutral because the number of protons is equal to the number of electrons.

(b) Rutherford could not explain the structural stability of an atom.

(a) According to the modern atomic model, what are the two main parts of which an atom is made of ?

(b) Where is the nucleus of an atom situated ?

(c) What are energy levels or shells of an atom ?

Answer

(a) An atom is made of:

1. The nucleus
2. The orbits or the shells present in the empty space that surrounds the nucleus.

(b) The nucleus is present in the central part of an atom.

(c) Orbits or shells are the imaginary paths traced by electrons in the empty space surrounding the nucleus. Each orbit is associated with a fixed amount of energy; hence, these orbits are also called energy levels or shells.

(a) State the rule according to which electrons are filled in various energy levels.

(b) Give the number of electrons that can be present in the first four shells of an atom using this rule.

(c) What is the maximum number of electrons present in the outermost (valence) shell ?

Answer

(a) According to the Bohr-Bury scheme, electrons are filled in different shells in a step-wise manner. The maximum number of electrons in a shell is given by the formula 2n², where n denotes the number of the shell. Also, electrons are not accommodated in an outer shell unless the inner shells are filled.

(b) The maximum number of electrons in each shell or orbit is determined by the formula 2n², where n is the number of shell.

K shell, n = 1, number of electrons = 2 x 1² = 2
L shell, n = 2, number of electrons = 2 x 2² = 8
M shell, n = 3, number of electrons = 2 x 3² = 18
N shell, n = 4, number of electrons = 2 x 4² = 32

(c) The maximum number of electrons in the outermost shell is 8. However, if an atom has only one shell, the maximum number of electrons in that shell is 2.

What are isotopes ? How does the existence of isotopes contradict Dalton's atomic theory ?

Answer

Isotopes are the atoms of the same element with the same atomic number but a different mass number due to the difference in the number of neutrons in their nucleus.

According to Dalton's theory, atoms of an element are identical in all respects, such as size, mass, density and chemical properties. However, isotopes of an element have the same atomic number, i.e. the same number of protons and electrons, but different mass numbers due to different numbers of neutrons. Therefore, isotopes differ in their physical properties, such as density, melting point and boiling point. This contradicts Dalton's atomic theory.

What is valency ? Name two elements having variable valency and state their valencies.

Answer

Valency is the combining capacity of the atoms of an element with the atoms of other elements to form molecules. The valency of an element or radical is the number of hydrogen atoms that combine with or displace one atom of that element or radical.

Two elements having variable valency are iron and copper.

State the mass number, the atomic number, number of neutrons and electronic configuration of the following atoms.

(a) $^{12}_{6}\text{C}$

(b) $^{16}_{8}\text{O}$

(c) $^{19}_{9}\text{F}$

(d) $^{20}_{10}\text{Ne}$

(e) ₁₃Al²⁷

(f) ₁₇Cl³⁵

Also, draw atomic diagrams for each of them.

Answer

(a) $^{12}_{6}\text{C}$

Atomic number: 6

Mass number: 12

Number of neutrons: 12 - 6 = 6

Electronic configuration: 2, 4

Atomic diagram:

(b) $^{16}_{8}\text{O}$

Atomic number: 8

Mass number: 16

Number of neutrons: 16 - 8 = 8

Electronic configuration: 2, 6

Atomic diagram:

(c) $^{19}_{9}\text{F}$

Atomic number: 9

Mass number: 19

Number of neutrons: 19 - 9 = 10

Electronic configuration: 2, 7

Atomic diagram:

(d) $^{20}_{10}\text{Ne}$

Atomic number: 10

Mass number: 20

Number of neutrons: 20 - 10 = 10

Electronic configuration: 2, 8

Atomic diagram:

(e) ₁₃Al²⁷

Atomic number: 13

Mass number: 27

Number of neutrons: 27 - 13 = 14

Electronic configuration: 2, 8, 3

Atomic diagram:

(f) ₁₇Cl³⁵

Atomic number: 17

Mass number: 35

Number of neutrons: 35 - 17 = 18

Electronic configuration: 2, 8, 7

Atomic diagram:

Draw the diagrams representing the atomic structures of the following :

[At. No. N = 7, Ne = 10]

(a) Nitrogen

(b) Neon

Answer

(a) Atomic structure diagram of Nitrogen is given below:

(b) Atomic structure diagram of Neon is given below:

Complete the table below by identifying A, B, C, D, E and F.

Answer

An atom is the smallest particle of all kinds of matter, which consists of subatomic particles called as electrons, protons and neutrons. Atoms of an element exhibit all the properties of that element. It has been discovered that certain atoms of the same element exhibit identical chemical properties but different physical properties. Such atoms of an element are called "isotopes".
Isotopes of some elements are useful in various fields such as medical science for treating diseases, or to determine the age of fossils, or in nuclear reactors as a fuel, etc.

(a) Define an isotope.

(b) Name the subatomic particle due to which the isotopes of an element differ from one another.

(c) Why do the isotopes show the same chemical properties but different physical properties?

(d) How does the existence of isotopes contradict Dalton's atomic theory ?

(e) Name the element whose isotope is used :

(i) to treat goitre

(ii) to determine the age of fossils

(iii) in nuclear reactors.

Answer

(a) Isotopes are atoms of the same element that have the same number of protons (atomic number) but different numbers of neutrons, and hence different mass numbers.

(b) The subatomic particle due to which the isotopes of an element differ from one another is neutron.

(c) Isotopes show the same chemical properties because they have the same atomic number and, therefore, the same electronic configuration.

They show different physical properties because their mass numbers are different due to different numbers of neutrons.

(d) John Dalton stated that atoms of the same element are identical in all respects. The existence of isotopes shows that atoms of the same element can have different masses, so they are not identical.

(e) The elements whose isotopes are used are:

(i) Iodine (radioactive isotope like I-131)

(ii) Carbon (Carbon-14 dating)

(iii) Uranium (Uranium-235)