Class 9 Chemistry Atoms and Molecules worksheet

Everything around us is made up of tiny particles called atoms and molecules. Even though we cannot see them with our eyes, they are the basic building blocks of all matter. Understanding atoms and molecules helps us learn how substances are formed, how they interact, and why different materials have different properties.

What Are Atoms?

An atom is the smallest unit of an element that still has the properties of that element. It consists of three main particles:

• Protons (positively charged)
• Neutrons (neutral, no charge)
• Electrons (negatively charged)

Atoms are extremely small, and their size is measured in nanometers. Different elements have different types of atoms. For example, a gold atom is different from an oxygen atom because they have different numbers of protons, neutrons, and electrons.

What Are Molecules?

A molecule is formed when two or more atoms join together. The atoms in a molecule can be of the same element or different elements. For example:

• Oxygen gas (O₂) consists of two oxygen atoms bonded together.
• Water (H₂O) consists of two hydrogen atoms and one oxygen atom.

Molecules can be small, like H₂O, or very large, like proteins and DNA in our bodies.

Laws of Chemical Combination

Scientists have studied how atoms and molecules combine to form substances. Some important laws include:

1. Law of Conservation of Mass – Mass is neither created nor destroyed in a chemical reaction. The total mass of reactants equals the total mass of products.
2. Law of Constant Proportion – A compound always has the same elements in the same ratio by mass. For example, water (H₂O) always has 2 hydrogen atoms for every 1 oxygen atom.

Class 9 Chemistry Atoms and Molecules worksheet

Q1. What is the symbol for lead?

a) L
b) Le
c) Pb
d) Pu

Q2. How many electrons are in the ion of an element with atomic number 13?

a) 13
b) 12
c) 11
d) 10

Q3. What is an ion?

Q4. What is the Avogadro constant?

Q5. The atomic number of three elements A, B, and C are 9, 10, and 13. Which one will form a positive ion (cation)?

Q6. The formula of a compound is A₃B₂. What is the valency of element B?

a) 2
b) 3
c) 1
d) Cannot be determined

Q7. What is the chemical formula of the following compounds?

a) Magnesium chloride
b) Calcium oxide
c) Copper nitrate
d) Aluminium chloride

Q8. When atoms of the same element combine, what do they form?

a) Molecules
b) Ions
c) Atoms
d) Compounds

Q9. The discovery of which particle proved that atoms are divisible?

a) Protons
b) Electrons
c) Neutrons
d) All of the above

Q10. Define atomic mass.

Q11. Explain the meaning of “the relative atomic mass of oxygen is 16.”

Q12. What is the difference between 2N and N₂?

Q13. What are the drawbacks of Dalton’s atomic theory?

Q14. State the Law of Conservation of Mass.

Q15. Give an example where the Law of Conservation of Mass applies to a physical change.

Q16. What is the molar mass of a substance? In which unit is it expressed?

Q17. Calculate the molar masses of the following:

a) Ozone (O₃)
b) Ethanoic acid (CH₃COOH)

Q18. Calculate the molecular masses of these compounds (use atomic masses: C = 12, H = 1, Cl = 35.5, S = 32, O = 16, Na = 23):

a) Chloroform
b) Sulphuric acid
c) Sodium hydroxide

Q19. Fill in the blanks:

a) Groups of atoms that act as a single ion are called ___ ions.
b) A chemical formula is also called a ___.
c) The valency of an ion is ___ to its charge.
d) The mass of 1 mole of a substance is called ___.
e) The formula mass of Na₂O is ___.

Q20. How many grams of oxygen gas contain the same number of molecules as 16g of sulfur dioxide (SO₂)? (Atomic masses: O = 16, S = 32)

Q21. Write the symbols for the following elements:

a) Copper
b) Mercury
c) Iron
d) Silver
e) Gold
f) Argon
g) Zinc

Q22. How many molecules are there in 4g of oxygen?

Q23. How many atoms are present in 1 mg of the following substances?

a) Butane (C₄H₁₀)

b) Nitrogen gas (N₂)

c) Sodium (Na)

d) Water (H₂O)

Q24. What will be the volume of 7 g of nitrogen gas (N₂) at NTP?

a) 11.2 L

b) 22.4 L

c) 5.6 L

d) 6.5 L

Q25. How many molecules are present in 0.18 g of water (H₂O)?

a) 0.18 × 6.023 × 10²³

b) 18 × 6.023 × 10²³

c) 6.023 × 10²²

d) 6.023 × 10²¹

Q26. What is the total number of protons, neutrons, and electrons in 12 g of carbon-12 (C-12)?

a) 1.084 × 10²⁵

b) 6.022 × 10²³

c) 6.023 × 10²²

d) 18

Q27. Which statement from Dalton’s atomic theory explains the law of conservation of mass?

a) Atoms cannot be divided.

b) All matter is made of tiny particles called atoms.

c) Elements combine in fixed ratios.

d) Atoms cannot be created or destroyed.

Q28. What will be the volume of 44 g of CO₂ at NTP?

a) 44 mL

b) 44 L

c) 22.4 mL

d) 22.4 L

Q29. A piece of sodium weighs 0.023 g. How many atoms does it contain?

a) 6.023 × 10²⁰

b) 60.23 × 10²²

c) 6.023 × 10²¹

d) 6.023 × 10¹⁹

Q30. 4.4 g of CO₂ and 2.24 L of H₂ (at STP) are mixed together. What is the total number of molecules in the container?

a) 6.023 × 10²³

b) 1.2044 × 10²³

c) 2 moles

d) 6.023 × 10²⁴

Q31. The number of atoms present in a molecule of an element is called its:

a) Molecularity

b) Atomicity

c) Valency

d) Reactivity

Q32. What is the difference between a molecule of an element and a molecule of a compound? Give an example of each.

Q33. What is the chemical formula of a compound formed between element X (valency 4) and element Y (valency 1)?

Q34. What do we call particles that have:

Q35. More electrons than a normal atom?

Q36. Fewer electrons than a normal atom?

Q37. Write the names of the following compounds along with the symbols and formulas of the ions present in them:

a) CuSO₄

b) (NH₄)₂SO₄

c) Na₂O

d) Na₂CO₃

e) CaCl₂

Q38. What is the mass of 0.2 moles of oxygen atoms?

Q39. Calculate the number of aluminium ions in 0.051 g of aluminium oxide (Al₂O₃).
(Atomic masses: Al = 27 u, O = 16 u)

Q40. What mass of oxygen will contain the same number of molecules as 56 g of nitrogen gas?
(Atomic masses: O = 16 u, N = 14 u)