Class 9 Science Chapter 9 Atomic Foundations of Matter – Extra Questions with Answers

The laws of chemical combination and Dalton’s atomic theory set up how chemical formulae get written in Chapter 9, and these extra questions test the criss-cross method and the difference between ionic and covalent bonding.

Last Updated: September 23, 2026

Very Short Answer Questions (1 mark)

Q1. State the law of conservation of mass in one line.
Ans: Mass can neither be created nor destroyed in a chemical reaction.

Q2. Who proposed the atomic theory based on the laws of chemical combination?
Ans: John Dalton.

Q3. Name the method used to write chemical formulae using valency.
Ans: Criss-cross method.

Q4. Do ionic compounds form by sharing or transferring electrons?
Ans: Transferring electrons.

Q5. What is molecular mass?
Ans: The sum of the atomic masses of all atoms in a molecule.

Short Answer Questions (2–3 marks)

Q6. Use the criss-cross method to write the chemical formula for aluminium oxide, given Al has valency 3 and oxygen has valency 2.
Ans: Al’s valency (3) becomes the subscript for O, and O’s valency (2) becomes the subscript for Al, giving Al₂O₃ (aluminium oxide).

Q7. Explain why formula unit mass, not molecular mass, is used for ionic compounds like sodium chloride.
Ans: Ionic compounds like NaCl don’t exist as discrete, individual molecules — they form a continuous lattice of oppositely charged ions. So instead of “molecular mass,” we calculate the formula unit mass, which sums the atomic masses of atoms in the simplest whole-number ratio (formula unit) representing the compound.

Q8. A 10 g sample of water is decomposed into hydrogen and oxygen. Using the law of conservation of mass, what should be true about the combined mass of hydrogen and oxygen produced?
Ans: According to the law of conservation of mass, the combined mass of hydrogen and oxygen produced must equal the original mass of water decomposed, i.e. 10 g total (in the fixed 1:8 ratio by mass for hydrogen to oxygen).

Higher-Order Thinking / Application Questions

Q9. A student claims that since 2 g of hydrogen combines with 16 g of oxygen to form water in one experiment, and 4 g of hydrogen combines with 32 g of oxygen to form water in another, the law of constant proportions is violated because the amounts are different. Explain why the student is mistaken.
Ans: The student is mistaken because the law of constant proportions concerns the ratio of elements combining, not the absolute amounts. In both experiments, the ratio of hydrogen to oxygen is 1:8 (2:16 and 4:32 both simplify to 1:8), so the law is upheld — only the total quantity of water produced differs, not the fixed proportion of the elements within it.

Q10. Calcium has valency 2, and carbonate (a polyatomic ion) has valency 2 (as a group). Determine the chemical formula for calcium carbonate, and calculate its formula unit mass given atomic masses Ca=40, C=12, O=16.
Ans: Since both valencies are 2, they cancel in the criss-cross method (ratio simplifies to 1:1), giving the formula CaCO₃. Formula unit mass = 40 (Ca) + 12 (C) + 3×16 (O₃) = 40 + 12 + 48 = 100.

Frequently Asked Questions

Why do atoms of the same element always have the same number of protons, even if the number of neutrons varies?
The number of protons, called the atomic number, defines what element an atom is, so while neutrons can vary to form isotopes, protons must remain the same for the atom to remain the same element.

How is the mass number of an atom calculated, and why does it differ from the atomic mass listed on the periodic table?
The mass number is the sum of protons and neutrons in a single atom, while atomic mass on the periodic table is a weighted average of the mass numbers of all naturally occurring isotopes, often a decimal.

Written by Satish

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