Atomic Foundations of Matter

Water can be obtained from various sources. Are all these samples of water chemically identical?

Answer

Yes, all pure samples of water are chemically identical, irrespective of their source. When water collected from any source — rivers, borewells or the ocean — is purified and analysed, it is always found to contain hydrogen and oxygen in a fixed mass ratio of 1 : 8. This is in accordance with the Law of Constant Proportions. Hence, every pure sample of water has the same composition and is chemically identical.

Oxygen is sometimes represented as O and sometimes as O₂. What is the difference between these symbols?

Answer

O represents a single atom of oxygen, whereas O₂ represents a molecule of oxygen made up of two oxygen atoms joined together by a double bond. Oxygen exists as a stable gas in the form of O₂, the molecule that is capable of independent existence.

Why does dissolved salt in water conduct electricity, but sugar does not?

Answer

Salt (sodium chloride) is an ionic compound. When it dissolves in water, it dissociates into free-moving ions (Na⁺ and Cl^–). These free ions carry the electric current, so the salt solution conducts electricity.

Sugar is a covalent compound. Although it is soluble in water, it does not produce any ions in the solution. As there are no free-moving charged particles, a sugar solution does not conduct electricity.

A student burns 10 g of ethanol in an open beaker. After the reaction, no residue is left in the beaker. Does this mean the Law of Conservation of Mass is violated? Explain.

Answer

No, the Law of Conservation of Mass is not violated. When ethanol burns, it combines with oxygen from the air to form carbon dioxide and water vapour, both of which are gases. In an open beaker, these gaseous products escape into the air, so no residue is left behind.

If the reaction were carried out in a closed container and the mass of the oxygen consumed and the gaseous products formed were also taken into account, the total mass of the reactants would be found equal to the total mass of the products. Hence, mass is conserved.

When 20 g of hydrogen reacts completely with 160 g of oxygen, how much water is formed according to the Law of Conservation of Mass?

Answer

According to the Law of Conservation of Mass, the total mass of the products equals the total mass of the reactants.

Mass of water formed = Mass of hydrogen + Mass of oxygen

= 20 g + 160 g = 180 g

Hence, 180 g of water is formed.

A compound consists of 40% sulfur and 60% oxygen by mass. In a sample of the same compound containing 20 g of sulfur, what mass of oxygen must be present to satisfy the Law of Constant Proportions?

Answer

In the compound, the mass ratio of sulfur to oxygen is fixed.

Ratio of sulfur : oxygen = 40 : 60

So, for every 40 g of sulfur there are 60 g of oxygen.

Mass of oxygen for 20 g of sulfur = $\dfrac{60}{40} \times 20$ = 30 g

Hence, 30 g of oxygen must be present to satisfy the Law of Constant Proportions.

Carbon monoxide (CO) contains carbon and oxygen in the mass ratio of 3:4. How much oxygen will combine with 9 g of carbon to form carbon monoxide?

Answer

In carbon monoxide, the mass ratio of carbon to oxygen is fixed at 3 : 4.

So, 3 g of carbon combines with 4 g of oxygen.

Mass of oxygen combining with 9 g of carbon = $\dfrac{4}{3} \times 9$ = 12 g

Hence, 12 g of oxygen will combine with 9 g of carbon.

The Law of Definite Proportions holds true for compounds but not for mixtures. Give reason.

Answer

In a compound, the elements are chemically combined in a fixed proportion by mass. Therefore, the ratio of the masses of its constituent elements always remains the same, no matter how or from where the compound is obtained. This satisfies the Law of Definite Proportions.

In a mixture, the components are merely mixed together physically and are not chemically combined. They can be present in any proportion. Since there is no fixed mass ratio, the Law of Definite Proportions does not apply to mixtures.

Students X and Y, both prepared an oxide of copper by combining copper and oxygen in the ratios of 4:1 and 8:2, respectively. Do their results justify the Law of Constant Proportions? Explain.

Answer

Yes, their results justify the Law of Constant Proportions.

For Student X, the ratio of copper to oxygen = 4 : 1

For Student Y, the ratio of copper to oxygen = 8 : 2 = 4 : 1

Both ratios reduce to the same ratio (4 : 1). Since the same compound contains copper and oxygen in the same fixed mass ratio, irrespective of who prepared it, the results justify the Law of Constant Proportions.

Assertion (A): 2 g of hydrogen combines with 16 g of oxygen to form 18 g of water.

Reason (R): According to Dalton's Atomic Theory, atoms combine in a simple whole number ratio by mass to form compounds.

Choose the correct option:

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

In water, hydrogen and oxygen are present in the fixed mass ratio of 1 : 8. So, 2 g of hydrogen combines with 16 g of oxygen (2 : 16 = 1 : 8) to form 18 g of water (2 g + 16 g = 18 g). Hence, A is true.

According to Dalton's Atomic Theory, atoms combine in a simple whole number ratio to form compounds. In water, two hydrogen atoms combine with one oxygen atom (a simple 2 : 1 ratio), which is why hydrogen and oxygen always combine in the fixed proportion shown in the assertion. Thus, R is true and it correctly explains A.

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

Nitrogen has five valence electrons. Draw the structure of the nitrogen molecule (N₂).

Answer

The structure of the nitrogen molecule is shown below:

The atomic number of fluorine is 9. Explain the formation of the fluorine molecule (F₂).

Answer

The electronic configuration of fluorine (atomic number 9) is 2, 7. It has 7 electrons in its valence shell and needs only 1 more electron to complete its octet.

To become stable, two fluorine atoms share one pair of electrons (one electron each). This forms a single covalent bond between them, giving the fluorine molecule (F₂), which is represented as F—F.

Show the formation of the following molecules:

(i) Carbon dioxide (CO₂)

(ii) Hydrogen sulfide (H₂S)

(iii) Ammonia (NH₃)

Answer

(i) Carbon dioxide (CO₂)

(ii) Hydrogen sulfide (H₂S)

(iii) Ammonia (NH₃)

Neon (atomic number 10) neither transfers nor shares its valence electrons. Explain.

Answer

The electronic configuration of neon (atomic number 10) is 2, 8. Its outermost (valence) shell already has 8 electrons, i.e., it has a complete octet.

Since neon already has a stable electronic configuration, it has no tendency to lose, gain or share electrons. That is why neon neither transfers nor shares its valence electrons and exists as a stable, inert atom.

What kind of ion will oxygen (O) form?

Answer

The electronic configuration of oxygen (atomic number 8) is 2, 6. It has 6 electrons in its valence shell and needs 2 more electrons to complete its octet.

By gaining 2 electrons, it acquires two units of negative charge and forms an anion, the oxide ion O^2–.

Fill in the blanks.

Among magnesium and chlorine, magnesium atom can give two electrons to become Mg²⁺. However, chlorine can take only one electron to become ............... . Now, ............... ion of magnesium and ............... ions of chlorine combine to give magnesium chloride.

Answer

Among magnesium and chlorine, magnesium atom can give two electrons to become Mg²⁺. However, chlorine can take only one electron to become Cl^– (chloride ion). Now, one ion of magnesium and two ions of chlorine combine to give magnesium chloride (MgCl₂).

Show the formation of cations of potassium (K) and calcium (Ca) atoms, and the formation of their corresponding chlorides using diagrams.

Answer

Formation of potassium cation (K⁺):

The electronic configuration of potassium (atomic number 19) is 2, 8, 8, 1. It has 1 electron in its valence shell, which it loses to attain a stable configuration, forming the cation K⁺.

Formation of calcium cation (Ca²⁺):

The electronic configuration of calcium (atomic number 20) is 2, 8, 8, 2. It has 2 electrons in its valence shell, which it loses to attain a stable configuration, forming the cation Ca²⁺.

Formation of potassium chloride (KCl):

Potassium loses 1 electron and chlorine gains that 1 electron. One K⁺ ion and one Cl^– ion combine to form KCl.

Formation of calcium chloride (CaCl₂):

Calcium loses 2 electrons, which are taken up by two chlorine atoms (one each). One Ca²⁺ ion and two Cl^– ions combine to form CaCl₂.

Illustrate how sodium sulfide (Na₂S) is formed.

Answer

The electronic configuration of sodium (atomic number 11) is 2, 8, 1. Each sodium atom loses 1 electron to form a Na⁺ ion.

The electronic configuration of sulfur (atomic number 16) is 2, 8, 6. The sulfur atom needs 2 electrons and gains them to form an S^2– ion.

Two sodium atoms each donate one electron to one sulfur atom. The two Na⁺ ions and one S^2– ion are held together by an ionic bond to form sodium sulfide (Na₂S).

Name the following:

(i) CO₂

(ii) NO₂

(iii) SF₆

(iv) PCl₃

Answer

(i) CO₂ — Carbon dioxide

(ii) NO₂ — Nitrogen dioxide

(iii) SF₆ — Sulfur hexafluoride

(iv) PCl₃ — Phosphorus trichloride

Write the formula for the following:

(i) Sodium hydrogencarbonate

(ii) Sulfur dioxide

(iii) Ferric chloride

(iv) Cuprous oxide

Answer

(i) Sodium hydrogencarbonate — NaHCO₃

(ii) Sulfur dioxide — SO₂

(iii) Ferric chloride — FeCl₃

(iv) Cuprous oxide — Cu₂O

Write the formulae for the compounds formed from the following pairs of ions:

(i) Fe³⁺ and OH^‒

(ii) K⁺ and CO₃^2–

Answer

(i) Fe³⁺ and OH^–

Crossing over the charges (3 and 1): the formula is Fe(OH)₃.

(ii) K⁺ and CO₃^2–

Crossing over the charges (1 and 2): the formula is K₂CO₃.

What type of chemical bond is present in a solid compound that does not conduct electricity in the solid state but conducts electricity when dissolved in water?

Answer

An ionic bond is present in such a compound.

Ionic compounds do not conduct electricity in the solid state because their ions are held in fixed positions by strong forces and cannot move. When the compound is dissolved in water, the ions become free to move, and the solution is then able to conduct electricity.

Metal M, with two electrons in its valence shell (M shell), reacts with oxygen to form a compound that is slightly soluble in water. Predict its:

(i) formula

(ii) type of bond

(iii) electrical conductivity of its aqueous solution.

Answer

The metal M has 2 electrons in its valence shell (the M shell), so its electronic configuration is 2, 8, 2. Its valency is 2, and it forms the cation M²⁺. Oxygen forms the anion O^2– (valency 2).

(i) Formula: Crossing over the charges (2 and 2) and simplifying gives MO.

(ii) Type of bond: It is an ionic bond, formed by the transfer of electrons from the metal to oxygen.

(iii) Electrical conductivity of its aqueous solution: Being an ionic compound, the portion that dissolves in water gives free-moving ions. Therefore, its aqueous solution conducts electricity.

Find the molecular mass of nitric acid (HNO₃).

Atomic mass — H = 1 u; N = 14 u; O = 16 u.

Answer

Molecular mass of HNO₃ = (1 u × 1) + (14 u × 1) + (16 u × 3)

= 1 u + 14 u + 48 u

= 63 u

Find the molecular mass of methane (CH₄).

Atomic mass — C = 12 u; H = 1 u.

Answer

Molecular mass of CH₄ = (12 u × 1) + (1 u × 4)

= 12 u + 4 u

= 16 u

Find the formula unit mass of potassium chloride (KCl).

Atomic mass — K = 39 u; Cl = 35.5 u.

Answer

Formula unit mass of KCl = (39 u × 1) + (35.5 u × 1)

= 39 u + 35.5 u

= 74.5 u

Find the formula unit mass of magnesium hydroxide, Mg(OH)₂.

Atomic mass — Mg = 24 u; O = 16 u; H = 1 u.

Answer

Formula unit mass of Mg(OH)₂ = (24 u × 1) + {(16 u × 1) + (1 u × 1)} × 2

= 24 u + (17 u × 2)

= 24 u + 34 u

= 58 u

You are given a chemical reaction in which zinc reacts with dilute hydrochloric acid to form zinc chloride and hydrogen gas.

Zinc + Hydrochloric acid (dilute) ⟶ Zinc chloride + Hydrogen

Design and perform an experiment to test the hypothesis that mass is conserved during the chemical reaction. You may use a set-up different from the one shown in Activity 9.2.

Answer

Since hydrogen gas is produced in this reaction, the experiment must be carried out in a closed system so that the gas formed does not escape.

Materials required: A conical flask, a small ignition tube (or a thin balloon), dilute hydrochloric acid, zinc granules, a thread, and a digital weighing balance.

Procedure:

1. Take about 20 mL of dilute hydrochloric acid in a clean, dry conical flask.

2. Place about 2 g of zinc granules in a small balloon, and fix the balloon tightly over the mouth of the conical flask using a thread, without allowing the zinc to fall into the acid.

3. Place the whole set-up on the digital weighing balance and record the initial mass.

4. Lift the balloon so that the zinc granules fall into the dilute hydrochloric acid.

5. A brisk effervescence is observed as hydrogen gas is produced, and the balloon inflates. The gas is trapped inside the balloon and cannot escape.

6. After the reaction is complete, place the set-up again on the balance and record the final mass.

Observation: The initial mass is equal to the final mass (within experimental error).

Conclusion: Since the total mass before the reaction equals the total mass after the reaction, mass is neither created nor destroyed. This verifies the Law of Conservation of Mass.

A particular element (A) has one electron in its third shell. There is another element (B) with six electrons in its second shell.

(i) How many electrons does A tend to give or take to become stable?

(ii) What kind of ion would it form?

(iii) How many electrons does B tend to give or take to become stable?

(iv) What kind of ion would it form?

(v) If A and B were to combine, what kind of bond would be formed?

(vi) What would be the formula for the compound thus formed?

Answer

Element A has one electron in its third shell, so its electronic configuration is 2, 8, 1. Element B has six electrons in its second shell, so its electronic configuration is 2, 6.

(i) A has 1 valence electron, so it tends to give (lose) 1 electron to become stable.

(ii) On losing 1 electron, A forms a cation (A⁺), a positively charged ion.

(iii) B has 6 valence electrons, so it tends to take (gain) 2 electrons to complete its octet.

(iv) On gaining 2 electrons, B forms an anion (B^2–), a negatively charged ion.

(v) Since A loses electrons (metal) and B gains electrons (non-metal), an ionic bond would be formed by the transfer of electrons.

(vi) A has valency 1 and B has valency 2. Crossing over the valencies gives the formula A₂B.

An element X has six electrons in its outer shell and forms a diatomic molecule.

(i) Why would that be so?

(ii) What kind of bond would it form?

(iii) Draw the structure of the molecule it would form.

(iv) A certain other element Y has two electrons in its second shell. Draw the structure of the molecule that X would form with Y.

Answer

Element X has six electrons in its outer shell (electronic configuration 2, 6). Element Y has two electrons in its second shell (electronic configuration 2, 2).

(i) X has 6 valence electrons and needs 2 more electrons to complete its octet. To achieve this, two atoms of X share two pairs of electrons with each other, so that both attain a stable octet. Hence, X exists as a diatomic molecule (X₂).

(ii) Since the two X atoms become stable by sharing electrons, they form a covalent bond (a double bond, as two pairs of electrons are shared).

(iii) The molecule X₂ has a double bond and is represented as X=X.

(iv) Y has 2 valence electrons (valency 2) and X needs 2 electrons (valency 2). Y transfers its 2 electrons to one X atom, forming Y²⁺ and X^2–. These oppositely charged ions are held together by an ionic bond, giving the compound YX.

You want to design a new ionic compound, where the total positive charge is 6+ and the total negative charge is 6–. Which of the following combinations gives the correct number of ions?

(i) 2 Al³⁺ and 3 Cl^–

(ii) 3 Mg²⁺ and 1 PO₄^3–

(iii) 2 Fe³⁺ and 3 O^2–

(iv) 3 Ca²⁺ and 2 SO₄^2–

Answer

We need the total positive charge = 6+ and the total negative charge = 6–.

(i) 2 Al³⁺ = 6+ ; 3 Cl^– = 3– → not balanced.

(ii) 3 Mg²⁺ = 6+ ; 1 PO₄^3– = 3– → not balanced.

(iii) 2 Fe³⁺ = 6+ ; 3 O^2– = 6– → balanced.

(iv) 3 Ca²⁺ = 6+ ; 2 SO₄^2– = 4– → not balanced.

Hence, the correct combination is (iii) 2 Fe³⁺ and 3 O^2–, which gives the compound Fe₂O₃.

Choose the correct statement(s) and correct the false statement(s).

(i) Elements are made up of molecules and compounds are made up of atoms.

(ii) The molecule of a compound is always made up of two or more atoms of the same kind.

(iii) One molecule of nitrogen gas contains three nitrogen atoms.

(iv) Water is made of two hydrogen atoms, covalently bonded with one oxygen atom.

Answer

(i) False. Both elements and compounds are made up of atoms. An element is made up of atoms of the same kind, while a compound is made up of atoms of two or more different elements combined in a fixed ratio.

(ii) False. The molecule of a compound is made up of two or more atoms of different kinds (different elements). A molecule made of atoms of the same kind is a molecule of an element.

(iii) False. One molecule of nitrogen gas (N₂) contains two nitrogen atoms.

(iv) True. Water (H₂O) is made of two hydrogen atoms covalently bonded with one oxygen atom.

Write the chemical formulae for the following compounds.

(i) Aluminium nitrate

(ii) Calcium oxide

(iii) Ferric oxide

Answer

(i) Aluminium nitrate — Al³⁺ and NO₃^– → Al(NO₃)₃

(ii) Calcium oxide — Ca²⁺ and O^2– → CaO

(iii) Ferric oxide — Fe³⁺ and O^2– → Fe₂O₃

Write the formulae of the compounds formed from the following pairs of ions.

(i) Ca²⁺ and Br^‒

(ii) Al³⁺ and CO₃^2–

(iii) K⁺ and SO₄^2–

(iv) NH₄⁺ and Cl^–

Answer

(i) Ca²⁺ and Br^– → CaBr₂

(ii) Al³⁺ and CO₃^2– → Al₂(CO₃)₃

(iii) K⁺ and SO₄^2– → K₂SO₄

(iv) NH₄⁺ and Cl^– → NH₄Cl

Which of the following, in Fig. 9.18, correctly represents Cl^– ion (Atomic number of chlorine = 17).

(i) (ii) (iii) (iv)

Answer

A neutral chlorine atom (atomic number 17) has the electronic configuration 2, 8, 7. To form the chloride ion (Cl^–), it gains one electron, giving it a total of 18 electrons.

The electronic configuration of the Cl^– ion is therefore 2, 8, 8 (a complete octet in the outermost shell).

Hence, the correct representation is the diagram showing the configuration 2, 8, 8 (18 electrons in all).

Determine the formula unit mass of the following substances.

(i) Ammonium nitrate (NH₄NO₃), used as a nitrogen fertiliser, which is essential for plant growth.

(ii) Phosphoric acid (H₃PO₄), used to make phosphate fertiliser and detergents.

(iii) Sodium hydrogencarbonate (NaHCO₃), used to relieve acidity and helps in digestion.

Answer

(Atomic masses: N = 14 u; H = 1 u; O = 16 u; P = 31 u; Na = 23 u; C = 12 u.)

(i) Ammonium nitrate (NH₄NO₃)

= (14 u × 1) + (1 u × 4) + (14 u × 1) + (16 u × 3)

= 14 u + 4 u + 14 u + 48 u

= 80 u

(ii) Phosphoric acid (H₃PO₄)

= (1 u × 3) + (31 u × 1) + (16 u × 4)

= 3 u + 31 u + 64 u

= 98 u

(iii) Sodium hydrogencarbonate (NaHCO₃)

= (23 u × 1) + (1 u × 1) + (12 u × 1) + (16 u × 3)

= 23 u + 1 u + 12 u + 48 u

= 84 u

Write the formulae for the compounds formed by the reaction of:

(i) Magnesium and nitrogen

(ii) Lithium and nitrogen

(iii) Sodium and sulfur

(iv) Aluminium and oxygen

Answer

(i) Magnesium and nitrogen — Mg²⁺ and N^3– → Mg₃N₂

(ii) Lithium and nitrogen — Li⁺ and N^3– → Li₃N

(iii) Sodium and sulfur — Na⁺ and S^2– → Na₂S

(iv) Aluminium and oxygen — Al³⁺ and O^2– → Al₂O₃

Complete the Table 9.3 by writing the formulae of the compounds formed by the cations on the left and the anions at the top. LiNO₃ is given as an example.

Table 9.3

Answer

5.3 g of sodium carbonate and 6.0 g of acetic acid react to produce 2.2 g of carbon dioxide, 0.9 g of water, and 8.2 g of sodium acetate. Verify whether the law of conservation of mass is valid.

Answer

Total mass of reactants = Mass of sodium carbonate + Mass of acetic acid

= 5.3 g + 6.0 g = 11.3 g

Total mass of products = Mass of carbon dioxide + Mass of water + Mass of sodium acetate

= 2.2 g + 0.9 g + 8.2 g = 11.3 g

Since the total mass of reactants (11.3 g) = total mass of products (11.3 g), the Law of Conservation of Mass is valid.

If a species has 11 protons, 12 neutrons and 10 electrons then

(i) what is its atomic number and mass number?

(ii) is it neutral, a cation or an anion? Explain.

(iii) write its electronic configuration.

(iv) name the species.

Answer

(i) Atomic number = number of protons = 11.

Mass number = number of protons + number of neutrons = 11 + 12 = 23.

(ii) The number of protons (11) is greater than the number of electrons (10). So, the species has one extra positive charge and is a cation (charge = +1).

(iii) The species has 10 electrons, its electronic configuration is 2, 8.

(iv) The element with atomic number 11 is sodium, and the cation with a +1 charge is the sodium ion (Na⁺).

Two elements, A and B, have the following configurations —

A: 2, 8, 5
B: 2, 8, 7

(i) Which element is more reactive?

(ii) Will A and B form ionic or covalent bonds when they combine? Explain using electron transfer or sharing.

(iii) Predict the formula of the compound they would form.

Answer

(i) B is more reactive. B has 7 valence electrons and needs only 1 electron to complete its octet, so it can attain stability easily. A has 5 valence electrons and needs 3 electrons, which is harder to achieve. Hence, B is more reactive than A.

(ii) Both A (5 valence electrons) and B (7 valence electrons) are non-metals and need to gain electrons. Neither can readily transfer electrons to the other, so they attain stability by sharing electrons. Therefore, A and B form covalent bonds.

(iii) A has a valency of 3 (needs 3 electrons) and B has a valency of 1 (needs 1 electron). So one A atom shares with three B atoms, giving the formula AB₃.

Assertion (A): Copper sulfate conducts electricity in the molten state but not in the solid state.

Reason (R): Copper and sulfate ions are fixed in the lattice in molten state, while in solid state they can move freely.

Choose the correct option:

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 because copper sulfate is an ionic compound. It conducts electricity in the molten state but not in the solid state.

R is false because the reason states the situation the wrong way round. In the solid state, the copper and sulfate ions are fixed in the lattice and cannot move, so the solid does not conduct electricity. In the molten state, the ions are free to move and therefore carry the electric current.

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

The species ²⁷Al, ⁸⁰Br^– and ²⁰¹Hg²⁺ have 13, 35 and 80 protons, respectively. How many electrons and neutrons do they have?

Answer

Number of neutrons = Mass number – Number of protons.

For ²⁷Al (13 protons, no charge):

Electrons = number of protons = 13

Neutrons = 27 – 13 = 14

For ⁸⁰Br^– (35 protons, charge –1):

Since it has gained 1 electron, Electrons = 35 + 1 = 36

Neutrons = 80 – 35 = 45

For ²⁰¹Hg²⁺ (80 protons, charge +2):

Since it has lost 2 electrons, Electrons = 80 – 2 = 78

Neutrons = 201 – 80 = 121