Chapter 3: Atoms and Molecules
Chapter 3: Atoms and Molecules Class 10 Science and Technology (212) NIOS Textbook Solutions
Chapter 3: ATOMS AND MOLECULES
INTEXT QUESTIONS 3.1
1. Name the scientists who proposed the law of conservation of mass and law of constant proportions.
Answer: The law of conservation of mass was proposed by Antoine Lavoisier.
The law of constant proportions was proposed by Joseph Proust.
2. 12 g of magnesium powder was ignited in a container having 20 g of pure oxygen. After the reaction was over, it was found that 12 g of oxygen was left unreacted. Show that it is according to law of constant proportions.
2Mg + O2→ 2MgO 
Answer: The chemical equation is : 2Mg + O2→ 2MgO
In container, 12g of Oxygen was left unreacted.
Therefore, amount of unreacted oxygen = (20 – 12)g = 08g.
Thus 12g of magnesium reacted with 8g of Oxygen in the ratio 12:8.
This is what we expected for MgO i.e 24g of Mg reacted with 16g of Oxygen or 12g of Mg will react with 8g of Oxygen.
Therefore, the observation obeys the law of constant proportions.
INTEXT QUESTIONS 3.2
1. Nitrogen forms three oxides : 
and 
. Show that it obeys law of multiple proportions.
Answer: Atomic mass of nitrogen is 14u and that of oxygen is 16u.
In NO, 14g of nitrogen reacted with 16g of oxygen
In NO2 , 14g of nitrogen reacted with 32g of oxygen
In N2O3 , 28g of nitrogen reacted with 48g of oxygen
Masses of oxygen combining with 14 g of nitrogen = 16g : 32g : 24g = 2 : 4 : 3
The masses of oxygen that combine with a fixed mass of nitrogen are in the ratio 2 : 4 : 3. Therefore, nitrogen oxides obey the law of multiple proportions.
2. Atomic number of silicon is 14. If there are three isotopes of silicon having 14, 15 and 16 neutrons in their nuclei, what would be the symbol of the isotope?
Answer: Atomic number of silicon = 14. So, number of protons = 14
We know that, Mass number = Number of protons + Number of neutrons
For the three isotopes:
(i) Neutrons = 14
A = 14 + 14 = 28
Symbol: 
(ii) Neutrons = 15
A = 14 + 15 = 29
Symbol: 
(iii) Neutrons = 16
A = 14 + 16 = 30
Symbol: 
Therefore, the three isotopes are , and .
3. Calculate molecular mass of the compounds whose formulas are provided below: 
and
Answer: Molecular mass of 
= 2 × 12u + 4 × 1u = 28u
Molecular mass of 
= 2 × 1u + 1 × 16u = 18u
Molecular mass of 
= 1 × 12u + 4 × 1u +1 × 16u = 32u
INTEXT QUESTIONS 3.3
1. Work out a relationship between number of molecules and mole.
Answer: The relationship between number of molecules and moles is,
1 mole of a substance 
molecules of that substance.
2. What is molecular mass? In what way it is different from the molar mass?
Answer: Molecular mass is the sum of atomic masses of all the atoms present in that molecule.
Molecular mass is the mass of one molecule whereas molar mass is the mass of 1 mol or 
elementary entities (atoms, molecules, ions)
3. Consider the reaction 
18 g of carbon was burnt in oxygen. How many moles of 
is produced?
Answer: The reaction is :
1 mol of C (12g) 1 mol of 
(32g) 1 mol of (44g)
12 g of carbon gives 1 mole of .
18g of carbon will give 1.5 mole of .
4. What is the molar mass of NaCl ?
Answer: Mass of Na is 23u and mass of Cl is 35.5u ( or g mol -1) .
Molar mass of NaCl = 23.0 + 35.5 gmol-1 = 58.5 g mol-1 
INTEXT QUESTIONS 3.4
1. Write the name of the expected compound formed between
(i) hydrogen and sulphur
(ii) nitrogen and hydrogen
(iii) magnesium and oxygen
Answer: The expected compounds formed are:
(i) Hydrogen + Sulphur → Hydrogen sulphide (H₂S)
(ii) Nitrogen + Hydrogen → Ammonia (NH₃)
(iii) Magnesium + Oxygen → Magnesium oxide (MgO)
2. Propose the formulas and names of the compounds formed between
(i) potassium and iodide ions
(ii) sodium and sulphate ions
(iii) aluminium and chloride ions
Answer: (i) Potassium ion (K⁺) + Iodide ion (I⁻)
K⁺ I⁻
Valency (ions) : 1 1
Formula: KI
Name: Potassium iodide
(ii) Sodium ion (Na⁺) + Sulphate ion (SO₄²⁻)
Na⁺ SO₄²⁻
Valency (ions) 1 2
Formula: Na₂SO₄
Name: Sodium sulphate
(iii) Aluminium ion (Al³⁺) + Chloride ion (Cl⁻)
Al³⁺ Cl⁻
Valency (ions) 3 1
Formula: AlCl₃
Name: Aluminium chloride
3. Write the formula of the compounds formed between
(i) 
and
(ii) 
and 
(iii) 
and 
Answer: (i) Hg²⁺ and Cl⁻
Hg²⁺ Cl⁻
Valency (ions) 2 1
Formula: HgCl₂
(ii) Pb²⁺ and PO₄³⁻
Pb²⁺ PO₄³⁻
Valency (ions): 2 3
Formula: Pb₃(PO₄)₂
(iii) Ba²⁺ and SO₄²⁻
Ba²⁺ SO₄²⁻
Valency (ions): 2 2
Formula: BaSO₄
TERMINAL EXERCISE
1. Describe the following:
(a) Law of conservation of mass
(b) Law of constant proportions
(c) Law of multiple proportions
Answer: (a) Law of Conservation of Mass :
This law states that mass can neither be created nor destroyed in a chemical reaction. The total mass of the reactants is always equal to the total mass of the products.
Example: If 12 g of carbon reacts with 32 g of oxygen, then the mass of carbon dioxide = 12 g + 32 g = 44g
(b) Law of Constant Proportions :
This law states that a sample of a pure substance always consists of the same elements combined in the same proportion by mass.
Example: Water (H₂O) always contains hydrogen and oxygen in the mass ratio = 1 : 8 .
(c) Law of Multiple Proportions:
This law states that when an element combines with another element and forms more than one compound, then different masses of the one element that combine with the fixed mass of another element are in the ratio of simple whole number or integer.
Example: Nitrogen and oxygen form NO and NO₂.
For 14 g of nitrogen:
In NO → Oxygen = 16 g
In NO₂ → Oxygen = 32 g
The ratio of NO and NO₂ = 16 : 32 = 1:2
Hence, it obeys the law of multiple proportions.
2. What is the atomic theory proposed by John Dalton? What changes have taken place in the theory during the last two centuries?
Answer: John Dalton proposed the atomic theory in the early 19th century to explain the nature of matter. The main points of his theory are:
(a) Matter consists of indivisible atoms.
(b) All the atoms of a given chemical element are identical in mass and in all other properties.
(c) Different chemical elements have different kinds of atoms and in particular such atoms have different masses.
(d) Atoms are indestructible and retain their identity in chemical reactions.
(e) The formation of a compound from its elements occurs through the combination of atoms of unlike elements in small whole number ratio.
Later scientific discoveries modified some parts of Dalton’s theory:
(a) Atoms are divisible: Scientists discovered smaller particles inside atoms such as electrons, protons, and neutrons.
(b) Atoms can be created and destroyed in nuclear reactions: This changed Dalton’s idea that atoms are indivisible and indestructible.
(c) Atoms of the same element may not be identical : Discovery of isotopes showed that atoms of the same element can have different masses.
(d) Atoms of different elements can have similar masses: Discovery of isobars showed atoms of different elements may have the same mass number.
(e) Atomic structure was discovered : Modern atomic theory explains that atoms contain a nucleus and electrons arranged around it.
3. Write the number of protons, neutrons and electron in each of the following isotopes : 
Answer: We know that, Neutrons = Mass number (A) − Atomic number (Z)
|
Isotope |
Atomic Number (Z) |
Mass Number (A) |
Protons |
Neutrons |
Electrons |
|
|
1 |
2 |
1 |
2 − 1 = 1 |
1 |
|
|
8 |
18 |
8 |
18 − 8 = 10 |
8 |
|
|
9 |
19 |
9 |
19 − 9 = 10 |
9 |
|
|
20 |
40 |
20 |
40 − 20 = 20 |
20 |
4. Boron has two isotopes with masses 10.13 u and 11.01 u and abundance of 19.77% and 80.23% respectively. What is the average atomic mass of boron? (Ans. 10.81 u)
Answer: Boron has two isotopes with masses 10.13 u and 11.01 u and abundance of 19.77% and 80.23% respectively.
The average atomic mass of boron 
5. Give symbol for each of the following isotopes:
(a) Atomic number 19, mass number 40
(b) Atomic number 7, mass number 15
(c) Atomic number 18, mass number 40
(d) Atomic number 17, mass number 37
Answer:
|
Atomic Number |
Mass number |
Element |
Isotope Symbol |
|
19 |
40 |
Potassium (K) |
|
|
7 |
15 |
Nitrogen (N) |
|
|
18 |
40 |
Argon (Ar) |
|
|
17 |
37 |
Chlorine (Cl) |
|
6. How does an element differ from a compound? Explain with suitable examples.
Answer: The difference between an element and a compound are :
|
Element |
Compound |
|
(i) An element is a pure substance made up of only one type of atom. |
(i) A compound is a pure substance formed when two or more elements combine chemically in a fixed ratio. |
|
(ii) It cannot be broken down into simpler substances by ordinary chemical methods. |
(ii) It can be broken down into simpler substances by chemical methods. |
|
(iii) It is represented by symbols. |
(iii) It is represented by chemical formulae. |
Examples:
Element: Oxygen contains only oxygen atoms.
Compound: Water (H₂O) contains hydrogen and oxygen chemically combined in a fixed ratio.
7. Charge of one electron is 
coulomb. What is the total charge on 1 mol of electrons?
Answer: The charge of one electron is given = 
C
Total charge 
C
8. How many molecules of 
are in 8.0 g of oxygen? If the molecules were completely split into O (Oxygen atoms), how many mole of atoms of oxygen would be obtained?
Answer: Given, Mass of oxygen gas = 8.0 g and Molar mass of = 32g/mol
Moles of 
mol
Molecules of 
molecules.
Moles of O atoms = 2 × 0.25 = 0.50 mol.
9. Assume that human body is 80% water. Calculate the number of molecules of water that are present in the body of a person whose weight is 65 kg.
Answer: Given, Body weight = 65 kg and Water content = 80%
Mass of water 
Molar mass of water 
g/mol
Moles of water 
mol
Molecules 
molecules of water.
10. Refer to atomic masses given in the Table (3.2) of this chapter. Calculate the molar masses of each of the following compounds : HCl , 
, 
, CO and NaCl
Answer: Here, H = 1 u, C = 12 u, N = 14 u, O = 16 u, Na = 23 u, Cl = 35.5 u
The molar mass of HCl = 1 × 1 + 1× 35.5 = 36.5 g/mol
The molar mass of NH3
= 1 × 14 + 3 × 1 = 17 g/mol
The molar mass of CH4
= 1× 12 + 4 × 1 = 18 g/mol
The molar mass of CO = 1 × 12 + 1× 16 = 28 g/mol
The molar mass of NaCl = 1 × 23 + 1 × 35.5 = 58.5 g/mol
11. Average atomic mass of carbon is 12.01 u. Find the number of moles of carbon in (a) 2.0 g of carbon. (b) 8.0 g of carbon.
Answer: Given, Average atomic mass of carbon = 12.01 u
(a) The number of moles of carbon 
mol
(b) The number of moles of carbon 
mol
12. Classify the following molecules as di, tri, tetra, penta and hexa atomic molecules: 
, 
, 
, 
, 
, 
, 
, HCl
Answer: The classification of molecules are :
|
Molecule |
Total number of atoms |
Classification |
|
H₂ |
2 (= 2H) |
Diatomic |
|
HCl |
2 (= 1H+1Cl) |
Diatomic |
|
SO₂ |
3 (= 1S+2O) |
Triatomic |
|
PCl₃ |
4 (= 1P+3Cl) |
Tetra-atomic |
|
P₄ |
4 (= 4P) |
Tetra-atomic |
|
SF₄ |
5 (= 1S+4F) |
Penta-atomic |
|
PCl₅ |
6 (= 1P+5Cl) |
Hexa-atomic |
|
CH₃OH |
6 (= 1C + 4H + 1O) |
Hexa-atomic |
13. What is the mass of
(a) 
atoms of oxygen
(b) 
molecules of 
(c) 
molecules of
Answer: (a) We have, 
oxygen atoms = 1 mole of O atoms
The molar mass of oxygen = 16 g/mol
The mass of oxygen atoms = 16 g
(b) We have, 
molecules = 1 mole of .
The molar mass of phosphorus = 31 g/mol
The mass of = 4 × 31 =124 g
(c) We have, number of moles 
The molar mass of = 32 g/mol
The mass of = 0.5 × 32 = 16 g
14. How many atoms are present in:
(a) 0.1 mol of sulphur
(b) 18 g of water ()
(c) 0.44 g of carbon dioxide ()
Answer: (i) The number of atom 
atoms
(ii) Molar mass of g/mol
Mole 
The number of atom 
atoms.
Total atoms 
atoms.
(iii) Molar mass of 
g/mol
Moles 
The number atoms 
atoms
Total atoms 
atoms
15. Write various postulates of Dalton’s atomic theory.
Answer: The various postulates of Dalton’s Atomic Theory are:
(i) Matter consists of indivisible atoms.
(ii) All the atoms of a given chemical element are identical in mass and in all other properties.
(iii) Different chemical elements have different kinds of atoms and in particular such atoms have different masses.
(iv) Atoms are indestructible and retain their identity in chemical reactions.
(v) The formation of a compound from its elements occurs through the combination of atoms of unlike elements in small whole number ratio.
16. Convert into mole:
(a) 16 g of oxygen gas ()
(b) 36 g of water ()
(c) 22 g of carbon dioxide ( )
Answer: (a) Molar mass of = 2 × 16 = 32 g/mol
Moles 
(b) Molar mass of = 2 × 1+ 1× 16 = 18 g/mol
Moles 
(c) Molar mass of = 1 × 12 + 2×16 = 12 + 32 =44 g/mol
Moles 
17. What does a chemical formula of a compound represents?
Answer: The chemical formula of a compound represents:
(i) The elements present in the compound.
(ii) The number of atoms of each element present in one molecule (or formula unit) of the compound.
(iii) The ratio in which the atoms are combined.
18. Write chemical formulas of the following compounds:
(a) Copper (II) sulphate (b) Calcium fluoride (c) Aluminium bromide (d) Zinc sulphate (e) Ammonium sulphate
Answer: The chemical formulas are:
(a) Copper (II) sulphate → CuSO₄
(b) Calcium fluoride → CaF₂
(c) Aluminium bromide → AlBr₃
(d) Zinc sulphate → ZnSO₄
(e) Ammonium sulphate → (NH₄)₂SO₄
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