Ace Your Science Exam: Quick Revision Notes on Atoms and Molecules

Looking for an easy way to revise ‘NCERT Atoms and Molecules Class 9‘ for your exams? This comprehensive guide breaks down key concepts like Dalton’s Atomic Theory, Laws of Chemical Combination, atomic mass, and chemical formulae. Perfect for quick exam prep and mastering the fundamentals of this important chapter!

Introduction to the Concept of Atoms (NCERT Atoms and Molecules Class 9)
Laws of Chemical Combination (NCERT Atoms and Molecules Class 9)
Dalton’s Atomic Theory (NCERT Atoms and Molecules Class 9)
Properties of Atoms (NCERT Atoms and Molecules Class 9)
Atomic Mass and Molecules (NCERT Atoms and Molecules Class 9)
Ions and Valency
Writing Chemical Formulae
Molecular and Formula Unit Mass
Key Takeaways for Exam
Questions and Answers on NCERT Atoms and Molecules Class 9
Exercises
MCQs on Atoms and Molecules Class 9 Science
Answer Key

Download PDF of Chapter 3 Atoms and Molecules Class 9 NCERT

Introduction to the Concept of Atoms (NCERT Atoms and Molecules Class 9)

Ancient Philosophies:
- Maharishi Kanad (India, ~500 BC): Proposed indivisible particles called Parmanu.
- Democritus (Greece): Coined “atom” (meaning indivisible).
Modern Foundation:
- Antoine Lavoisier and Joseph Proust established the Laws of Chemical Combination.

Laws of Chemical Combination (NCERT Atoms and Molecules Class 9)

Law of Conservation of Mass:
- Mass remains constant during chemical reactions.
Law of Constant Proportions:
- A compound always contains the same elements in fixed proportions by mass.

Dalton’s Atomic Theory (NCERT Atoms and Molecules Class 9)

All matter consists of tiny, indivisible atoms.
Atoms of a given element are identical in mass and properties.
Atoms combine in simple whole-number ratios to form compounds.
Atoms cannot be created or destroyed in chemical reactions.

Properties of Atoms (NCERT Atoms and Molecules Class 9)

Atoms are extremely small, with sizes measured in nanometers (nm).
Modern symbols for elements (e.g., H for Hydrogen, O for Oxygen) are standardised by IUPAC.

Atomic Mass and Molecules (NCERT Atoms and Molecules Class 9)

Atomic Mass Unit (u): 1/12th the mass of a carbon-12 atom.
Molecules are groups of two or more atoms held together by chemical bonds.
- E.g., Water (H₂O): Ratio of hydrogen to oxygen is always 2:1.

Ions and Valency

Ions:
- Cation (+ve charge), e.g., Na⁺.
- Anion (-ve charge), e.g., Cl⁻.
Valency:
- The combining capacity of an atom.
- Example: In H₂O, Hydrogen (valency 1), Oxygen (valency 2).

Writing Chemical Formulae

Use criss-cross method to balance valencies.
- E.g., Magnesium chloride: Mg²⁺ + Cl⁻ → MgCl₂.
Polyatomic ions (groups of atoms):
- E.g., Ammonium sulfate: (NH₄)₂SO₄.

Molecular and Formula Unit Mass

Molecular Mass: Sum of atomic masses in a molecule.
- E.g., H₂O = $2 × 1 + 16 = 18 u.$
Formula Unit Mass: Calculated for ionic compounds.
- E.g., NaCl = $23 + 35.5 = 58.5 u.$

Key Takeaways for Exam

Memorise Dalton’s Atomic Theory and the laws of chemical combination.
Practice writing chemical formulae using valencies.
Understand atomic and molecular masses for numerical problems.

Questions and Answers on NCERT Atoms and Molecules Class 9

1. In a reaction, 5.3 g of sodium carbonate reacted with 6 g of acetic acid. The products were 2.2 g of carbon dioxide, 0.9 g of water, and 8.2 g of sodium acetate. Show that these observations are in agreement with the law of conservation of mass.

Answer:
According to the law of conservation of mass, the total mass of reactants equals the total mass of products.

Total mass of reactants = 5.3 g (sodium carbonate) + 6 g (acetic acid) = 11.3 g
Total mass of products = 2.2 g (carbon dioxide) + 0.9 g (water) + 8.2 g (sodium acetate) = 11.3 g
Since the total mass of reactants equals the total mass of products, the observation agrees with the law of conservation of mass.

2. Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?

Answer:
The ratio of hydrogen to oxygen by mass is 1:8.

For 1 g of hydrogen, 8 g of oxygen is required.
For 3 g of hydrogen, the required oxygen is:
$3×8=24 g3 \times 8 = 24 \, \text{g}.$
So, 24 g of oxygen gas is required.

3. Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?

Answer:
The postulate is: “Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.”

4. Which postulate of Dalton’s atomic theory can explain the law of definite proportions?

Answer:
The postulate is: “Atoms of different elements combine in fixed, simple whole-number ratios to form compounds.”

5. Define the atomic mass unit.

Answer:
One atomic mass unit (u) is defined as one-twelfth (1/12th) of the mass of one carbon-12 atom.

6. Why is it not possible to see an atom with naked eyes?

Answer:
Atoms are extremely small, with sizes in the range of 10⁻¹⁰ meters. They are smaller than the resolution limit of the human eye.

7. Write down the formulae of:

(i) Sodium oxide → Na₂O
(ii) Aluminium chloride → AlCl₃
(iii) Sodium sulphide → Na₂S
(iv) Magnesium hydroxide → Mg(OH)₂

8. Write down the names of compounds represented by the following formulae:

(i) Al₂(SO₄)₃ → Aluminium sulfate
(ii) CaCl₂ → Calcium chloride
(iii) K₂SO₄ → Potassium sulfate
(iv) KNO₃ → Potassium nitrate
(v) CaCO₃ → Calcium carbonate

9. What is meant by the term chemical formula?

Answer:
A chemical formula is a symbolic representation of a compound, showing the elements present and the number of atoms of each element in the compound.

10. How many atoms are present in:

(i) H₂S molecule → 3 atoms (2 hydrogen + 1 sulfur)
(ii) PO₄³⁻ ion → 5 atoms (1 phosphorus + 4 oxygen)

11. Calculate the molecular masses of:

H₂ = $2×1=2 u2 \times 1 = 2 \, u$
O₂ = $2×16=32 u2 \times 16 = 32 \, u$
Cl₂ = $2×35.5=71 u2 \times 35.5 = 71 \, u$
CO₂ = $12+(2×16)=44 u12 + (2 \times 16) = 44 \, u$
CH₄ = $12+(4×1)=16 u12 + (4 \times 1) = 16 \, u$
C₂H₆ = $(2×12)+(6×1)=30 u(2 \times 12) + (6 \times 1) = 30 \, u$
C₂H₄ = $(2×12)+(4×1)=28 u(2 \times 12) + (4 \times 1) = 28 \, u$
NH₃ = $14+(3×1)=17 u14 + (3 \times 1) = 17 \, u$
CH₃OH = $12+(4×1)+16=32 u12 + (4 \times 1) + 16 = 32 \, u$

12. Calculate the formula unit masses of:

ZnO = $65+16=81 u65 + 16 = 81 \, u$
Na₂O = $(2×23)+16=62 u(2 \times 23) + 16 = 62 \, u$
K₂CO₃ = $(2×39)+12+(3×16)=138 u(2 \times 39) + 12 + (3 \times 16) = 138 \, u$

Exercises

1. Calculate the percentage composition of the compound by weight.

Mass of boron = $0.096 , \text{g}$
Mass of oxygen = $0.144 , \text{g}$
Total mass of the compound = $0.24 , \text{g}$

Percentage of boron: Percentage of boron= $0.0960.24×100=40%\text{Percentage of boron} = \frac{0.096}{0.24} \times 100 = 40\%$

Percentage of oxygen: Percentage of oxygen= $0.1440.24×100=60%\text{Percentage of oxygen} = \frac{0.144}{0.24} \times 100 = 60\%$

Thus, the compound is $40%$ boron and $60%$ oxygen by weight.

2. What mass of carbon dioxide will be formed when $3.00 , \text{g}$ of carbon is burnt in $50.00 , \text{g}$ of oxygen? Which law of chemical combination governs this?

Given:
Mass of carbon = $3.00 , \text{g}$
Mass of oxygen = $50.00 , \text{g}$

From the reaction, $3.00 , \text{g}$ of carbon reacts with $8.00 , \text{g}$ of oxygen to form $11.00 , \text{g}$ of carbon dioxide. Any excess oxygen remains unreacted.

Mass of carbon dioxide formed: Mass of CO₂= $\text{Mass of C}+\text{Mass of O}=3.00 \text{g}+8.00 \text{g}=11.00 \text{g}\text{Mass of CO}_2 = \text{Mass of C} + \text{Mass of O} = 3.00 \, \text{g} + 8.00 \, \text{g} = 11.00 \, \text{g}$

Thus, when $3.00 , \text{g}$ of carbon is burnt in $50.00 , \text{g}$ of oxygen, only $11.00 , \text{g}$ of carbon dioxide is formed, and $42.00 , \text{g}$ of oxygen remains unreacted.

This observation follows the law of conservation of mass, which states that mass can neither be created nor destroyed in a chemical reaction.

3. What are polyatomic ions? Give examples.

Polyatomic ions are ions that consist of two or more atoms bonded together and carry a net charge.

Examples:

Ammonium ion: $\text{NH}_4^+$
Hydroxide ion: $\text{OH}^-$
Carbonate ion: $\text{CO}_3^{2-}$
Sulphate ion: $\text{SO}_4^{2-}$

4. Write the chemical formulae of the following:

(a) Magnesium chloride: $\text{MgCl}_2$
(b) Calcium oxide: $\text{CaO}$
(c) Copper nitrate: $\text{Cu(NO}_3\text{)}_2$
(d) Aluminium chloride: $\text{AlCl}_3$
(e) Calcium carbonate: $\text{CaCO}_3$

5. Give the names of the elements present in the following compounds:

(a) Quick lime ( $\text{CaO}$ ): Calcium and oxygen
(b) Hydrogen bromide ( $\text{HBr}$ ): Hydrogen and bromine
(c) Baking powder ( $\text{NaHCO}_3$ ): Sodium, hydrogen, carbon, and oxygen
(d) Potassium sulphate ( $\text{K}_2\text{SO}_4$ ): Potassium, sulphur, and oxygen

6. Calculate the molar mass of the following substances:

Given atomic masses: H = $1 , \text{u}$ , C = $12 , \text{u}$ , O = $16 , \text{u}$ , S = $32 , \text{u}$ , P = $31 , \text{u}$ , Cl = $35.5 , \text{u}$

(a) Ethyne ( $\text{C}_2\text{H}_2$ ): Molar mass= $2×12+2×1=26 u\text{Molar mass} = 2 \times 12 + 2 \times 1 = 26 \, \text{u}$

(b) Sulphur molecule ( $\text{S}_8$ ): Molar mass= $8×32=256 u\text{Molar mass} = 8 \times 32 = 256 \, \text{u}$

(d) Hydrochloric acid ( $\text{HCl}$ ): Molar mass= $1+35.5=36.5 u\text{Molar mass} = 1 + 35.5 = 36.5 \, \text{u}$

(e) Nitric acid ( $\text{HNO}_3$ ): Molar mass= $1+14+3×16=63 u\text{Molar mass} = 1 + 14 + 3 \times 16 = 63 \, \text{u}$

Here is a list of 20 multiple-choice questions (MCQs) based on the “Atoms and Molecules” chapter of the Class 9 NCERT Science textbook. These questions cover key concepts and ensure a thorough understanding of the chapter. The answer key is provided at the end.

MCQs on Atoms and Molecules Class 9 Science

What does the law of conservation of mass state?
a) Matter can be created but not destroyed
b) Matter can be destroyed but not created
c) Mass can neither be created nor destroyed in a chemical reaction
d) Mass is always variable in chemical reactions
Who proposed the law of constant proportions?
a) Antoine Lavoisier
b) Joseph Proust
c) John Dalton
d) Democritus
The ratio of hydrogen to oxygen by mass in water is:
a) 1:8
b) 1:16
c) 2:16
d) 1:4
Which of the following is a polyatomic ion?
a) $\text{Na}^+$
b) $\text{OH}^-$
c) $\text{Cl}^-$
d) $\text{Ca}^{2+}$
The smallest particle of an element that participates in chemical reactions is called:
a) A molecule
b) A proton
c) An atom
d) An ion
Which postulate of Dalton’s atomic theory explains the law of conservation of mass?
a) Atoms combine in simple whole numbers to form compounds
b) Atoms cannot be created or destroyed in a chemical reaction
c) Atoms are indivisible
d) Atoms of the same element are identical
What is the relative atomic mass of oxygen?
a) 1 u
b) 16 u
c) 12 u
d) 14 u
What is the chemical formula of magnesium hydroxide?
a) $\text{MgOH}_2$
b) $\text{Mg(OH)}_2$
c) $\text{Mg}_2\text{OH}$
d) $\text{Mg}_2\text{(OH)}$
If 6 g of carbon reacts with 16 g of oxygen, the mass of carbon dioxide produced will be:
a) 6 g
b) 16 g
c) 22 g
d) 32 g
What does a chemical formula represent?
a) The number of moles in a compound
b) The valency of the compound
c) The composition of a compound
d) The type of ions in the compound
Which of the following molecules is diatomic?
a) $\text{H}_2$
b) $\text{CH}_4$
c) $\text{P}_4$
d) $\text{S}_8$
What is the atomicity of sulphur in $\text{S}_8$ ?
a) 1
b) 2
c) 4
d) 8
The chemical name of $\text{CaCO}_3$ is:
a) Calcium carbonate
b) Calcium oxide
c) Calcium hydroxide
d) Calcium chloride
What is the formula unit mass of sodium chloride ( $\text{NaCl}$ )?
a) 58.5 u
b) 22.5 u
c) 36.5 u
d) 44.5 u
A molecule of oxygen gas ( $\text{O}_2$ ) has a molecular mass of:
a) 16 u
b) 32 u
c) 12 u
d) 8 u
One atomic mass unit (1 u) is equal to:
a) $\frac{1}{2}$ the mass of an oxygen atom
b) $\frac{1}{16}$ the mass of an oxygen atom
c) $\frac{1}{12}$ the mass of a carbon-12 atom
d) The mass of one hydrogen atom
What type of charge does a carbonate ion ( $\text{CO}_3$ ) carry?
a) $1^+$
b) $1^-$
c) $2^+$
d) $2^-$
Which element forms a monoatomic molecule?
a) Nitrogen
b) Helium
c) Oxygen
d) Chlorine
What is the molecular mass of $\text{CH}_3\text{OH}$ (methanol)?
a) 28 u
b) 30 u
c) 32 u
d) 34 u
How many hydrogen atoms are present in a molecule of $\text{NH}_3$ ?
a) 2
b) 3
c) 1
d) 4