Atoms and Molecules Class 9 Worksheet Science Chapter 3

Q.1. The maximum number of electrons in L shell is
(a) 8
(b) 18
(c) 28
(d) 38 

Correct Answer is Option (a)

Q.2. Which of the following statements is correct?
(a) Cathode rays travel in straight line and have momentum.
(b) Cathode rays travel in straight line and have no momentum
(c) Cathode rays do not travel in straight line but have Momentum.
(d) Cathode rays do not travel in straight line and have no momentum. 

Correct Answer is Option (a)

Q.3. Oxygen is – 
(a) Monovalent
(b) Bivalent
(c) Trivalent
(d) Tetravalent

Correct Answer is Option (b)

Q.4. Molecular mass of water (H₂O) is
(a) 18g
(b) 8g
(c) 33g
(d) 34g

Correct Answer is Option (a)

Q.5. Atomicity of chlorine and Argon is
(a) Diatomic and Monoatomic
(b) Monoatomic and Diatomic
(c) Monoatomic and Monoatomic
(d) Diatomic and Diatomic

Correct Answer is Option (a)

Q.6. Stat the properties of cathode rays? 

Properties of cathode rays are:
(1) They travel in straight line
(2) They have momentum and energy
(3) They are deflected by electric and magnetic fields.

Q.7. Define the terms:
(a) Atomic number
(b) Mass number 

Atomic Number is defined as the total number of protons present in an atom. Mass number is defined as the sum total of number of protons and the number of neutrons present in an atom.

Q.8. State the law of constant Proportion? 

According to law of constant proportion, whatever the method of its formation, a chemical compound in its pure state will always contain the same elements combined together in the fixed ratio by mass.

Q.9. Which postulate of Dalton’s atomic theory can explain the law of definite proportions?

The postulate of Dalton’s atomic theory which explains the law of definite proportions is “Atoms combine in the ratio of small whole numbers to form compounds and the relative number and kinds of atoms are constant in a given compound.”

Q.10. Which element will be more reactive and why → the element whose atomic number is 10 or the one whose atomic number is 11?

Element with atomic number 11 is more reactive than the one with atomic number 10 because electronic configuration of atomic number 11 will be 2, 8, 1 so, it has to loose only 1e- from its outermost shall to be stable which is more easy than the element with atomic number 10 because its electronic configuration is 2, 8 and has 8e- in the outermost shell and hence is already stable.

Q.11. What is the mass of 0.2 mole of oxygen atoms?

0.2 mole of oxygen atoms
Mass of 1 mole of oxygen (O) atoms = 16 u
Mass of 0.2 mole of oxygen (O) atoms = 0.2 × 16 = 3.2 u

Q.12. The mass of an atom of element (X) is 2.0 × 10^–23 g. Calculate its atomic mass.

One atom of element (X) has mass = 2.0 × 10^–23 g.
6.022 × 10²³ atoms of element (X) will have mass 
= (2.0 × 10^–23 g) × (6.022 × 10²³) 
= 12.044 ≈ 12.0 g

Q.13. Calculate the molar mass of Nitric acid.

Chemical formula of Nitric acid is (HNO₃)
Molar mass of HNO₃ = (1 × atomic mass of H) + (1 × atomic mass of N) + (3 × atomic mass of O) 
= (1 × 1u) + (1 × 14u) + (3 × 16u) = 63u

Q.14. Explain why the number of atoms in one mole of hydrogen gas is double the number of atoms in one mole of helium gas.

Hydrogen gas is diatomic in nature (H₂) while helium gas is monoatomic (He). As a result, the number of atoms in one mole of hydrogen is (2 × N_A) which is double as compared to number of atoms in one mole of helium (N_A)

Q.15. If the valency of carbon is 4 and that of sulphur is 2, what is the chemical formula and name of the compound formed between carbon and sulphur atoms?

The chemical formula of compound can be written by exchanging the valencies (cross-over). Therefore, the expected formula is C₂S₄ or CS₂. The compound is called carbon disulphide.