Atoms are the smallest particles of elements that take part in chemical reactions. Important Questions Class 9 Science Chapter 9 cover Atomic Foundations of Matter, including laws of chemical combination, Dalton’s atomic theory, atoms, molecules, valency, chemical formulae, molecular mass, and formula unit mass.
A spoon of water, a grain of salt, and a piece of chalk all look simple, but each is made of tiny particles arranged in fixed ways. Class 9 Science Chapter 9 explains how atoms combine to form molecules and compounds. The chapter becomes easier when students understand three links clearly: laws explain mass ratios, valency explains formulae, and molecular mass numericals show how much each formula represents.
Key Takeaways
| Detail |
Information |
| Chapter |
Class 9 Science Chapter 9 |
| Topic |
Atomic Foundations of Matter |
| Syllabus |
CBSE 2026 |
| Question Types |
VSA, Short Answer, Long Answer, Numericals, Assertion-Reason, Case Study |
| Main Concepts |
Atoms, Molecules, Valency, Chemical Formulae, Atomic Theory |
| High-Value Topics |
Laws of Chemical Combination, Molecular Mass, Formula Unit Mass |
| Best Revision Method |
Learn laws first, then formula-writing, then numericals |
Class 9 Science Chapter List
Class 9 Science Chapter 9 Atomic Foundations of Matter: Topics Covered
Class 9 science chapter 9 important questions with answers usually test laws, formula-writing, valency, and mass calculations.
Revise these topics before solving the question bank.
- Laws of chemical combination
- Law of conservation of mass
- Law of definite proportions
- Dalton’s atomic theory
- Postulates and limitations of Dalton’s theory
- Atoms and atomic mass
- Molecular symbols and atomic symbols
- Molecules of elements and compounds
- Atomicity
- Valency and common radicals
- Writing chemical formulae
- Ionic and covalent bonds
- Molecular mass numericals
- Formula unit mass questions
- Assertion-reason and case-based questions
Important Questions Class 9 Science Chapter 9 with Answers
Important Questions Class 9 Science Chapter 9 should be revised in order. Start with definitions and laws, then move to Dalton’s theory, chemical formulae, numericals, assertion-reason, and case studies.
These class 9 science chapter 9 question answer sets cover direct questions, concept-based answers, and calculation-based Chemistry practice.
Very Short Answer Important Questions Class 9 Science Chapter 9
Very short answer questions test definitions, symbols, laws, and simple formulae.
Keep answers direct and use correct Chemistry terms.
Class 9 Science Chapter 9 Important Questions with Answers
Q1. What is an atom?
Ans. An atom is the smallest particle of an element that takes part in a chemical reaction.
It retains the chemical properties of that element.
Q2. State the Law of Conservation of Mass.
Ans. Mass is neither created nor destroyed in a chemical reaction.
The total mass of reactants is always equal to the total mass of products.
Q3. What does the symbol H represent?
Ans. H represents one atom of hydrogen.
It also represents one mole of hydrogen atoms when used in molar terms.
Q4. Define atomicity.
Ans. Atomicity is the number of atoms present in one molecule of an element.
Example: Ozone, O₃, has atomicity 3.
Q5. What is the valency of nitrogen?
Ans. Nitrogen has valency 3.
It usually forms three bonds to complete its octet.
Q6. Who proposed Dalton’s atomic theory?
Ans. John Dalton proposed the atomic theory in 1808.
Q7. What is a molecule?
Ans. A molecule is the smallest particle of an element or compound that can exist independently.
It shows the chemical properties of that substance.
Q8. Write the chemical formula for calcium oxide.
Ans. The chemical formula for calcium oxide is CaO.
Calcium has valency 2 and oxygen has valency 2, so the formula simplifies to CaO.
Short Answer Questions Class 9 Science Chapter 9
Short answer questions test laws, formula-writing, and differences between related concepts.
Use examples like water, carbon dioxide, sodium chloride, and magnesium chloride.
Atomic Foundations of Matter Class 9 Important Questions
Q1. State the Law of Definite Proportions with one example.
Ans. The Law of Definite Proportions states that a chemical compound always contains the same elements in the same proportion by mass.
Example: Water always contains hydrogen and oxygen in the mass ratio 1:8, whatever its source.
Q2. Write any two postulates of Dalton’s atomic theory.
Ans.
- All matter is made of tiny particles called atoms.
- Atoms of the same element are identical in mass and properties.
Atoms of different elements have different masses and properties.
Q3. Distinguish between atoms and molecules.
Ans.
| Atom |
Molecule |
| Smallest particle of an element |
Group of two or more atoms |
| May or may not exist independently |
Can exist independently |
| Takes part in chemical reactions |
Shows properties of the substance |
| Example: H |
Example: H₂ |
Q4. How do you write the chemical formula for magnesium chloride?
Ans. Magnesium has valency 2.
Chlorine has valency 1.
Using the criss-cross method:
Mg²⁺ and Cl⁻ give MgCl₂.
So, the formula is MgCl₂.
Q5. What is the difference between a molecule of an element and a molecule of a compound?
Ans. A molecule of an element contains atoms of the same element.
Example: O₂ is a molecule of oxygen.
A molecule of a compound contains atoms of different elements in a fixed ratio.
Example: H₂O is a molecule of water.
Q6. What are polyatomic ions? Give two examples.
Ans. Polyatomic ions are groups of atoms that carry a net charge and behave as one unit.
Examples:
- Ammonium ion: NH₄⁺
- Sulphate ion: SO₄²⁻
Long Answer Important Questions Class 9 Science Chapter 9
Long answers should include definition, explanation, examples, and limitations wherever needed.
Use proper terms such as atoms, compounds, fixed mass ratio, valency, and whole-number ratio.
Dalton Atomic Theory Questions Class 9
Q1. Explain Dalton’s atomic theory. What are its limitations?
Ans. Dalton’s atomic theory explains matter in terms of atoms.
Main postulates:
- All matter is made of tiny particles called atoms.
- Atoms are indivisible in chemical reactions.
- Atoms of the same element have the same mass and properties.
- Atoms of different elements have different masses and properties.
- Atoms combine in simple whole-number ratios to form compounds.
- Atoms are neither created nor destroyed in chemical reactions.
Limitations:
- Atoms are not indivisible. They contain protons, neutrons, and electrons.
- Atoms of the same element can have different masses. These are isotopes.
- Atoms of different elements can have the same mass. These are isobars.
- The theory does not explain why atoms combine.
Dalton’s theory was still important because it explained the laws of chemical combination.
Q2. Describe the method of writing chemical formulae with two examples.
Ans. Chemical formulae are written using symbols and valencies.
Steps:
- Write the symbols of the elements or radicals.
- Write their valencies below the symbols.
- Cross the valencies and use them as subscripts.
- Simplify the subscripts to the lowest whole-number ratio.
- Use brackets for polyatomic ions when more than one ion is needed.
Example 1: Sodium sulphate
Sodium = Na, valency 1
Sulphate = SO₄, valency 2
Formula = Na₂SO₄
Example 2: Aluminium oxide
Aluminium = Al, valency 3
Oxygen = O, valency 2
Formula = Al₂O₃
Class 9 Science Chapter 9 Numericals
Class 9 science chapter 9 numericals mostly use molecular mass and formula unit mass.
Read each subscript carefully before multiplying atomic masses.
Molecular Mass Numericals Class 9
Molecular mass is the sum of atomic masses of all atoms in one molecule.
Formula:
Molecular mass = Sum of atomic masses of all atoms
Q1. Calculate the molecular mass of H₂SO₄. Atomic masses: H = 1u, S = 32u, O = 16u.
Ans.
Molecular mass of H₂SO₄
= 2(1) + 1(32) + 4(16)
= 2 + 32 + 64
= 98u
Q2. Calculate the molecular mass of HNO₃. Atomic masses: H = 1u, N = 14u, O = 16u.
Ans.
Molecular mass of HNO₃
= 1(1) + 1(14) + 3(16)
= 1 + 14 + 48
= 63u
Q3. Calculate the molecular mass of C₆H₁₂O₆. Atomic masses: C = 12u, H = 1u, O = 16u.
Ans.
Molecular mass of C₆H₁₂O₆
= 6(12) + 12(1) + 6(16)
= 72 + 12 + 96
= 180u
Q4. Calculate the molecular mass of CO₂. Atomic masses: C = 12u, O = 16u.
Ans.
Molecular mass of CO₂
= 1(12) + 2(16)
= 12 + 32
= 44u
Q5. Calculate the molecular mass of NH₃. Atomic masses: N = 14u, H = 1u.
Ans.
Molecular mass of NH₃
= 1(14) + 3(1)
= 14 + 3
= 17u
Formula Unit Mass Questions Class 9
Formula unit mass is used for ionic compounds.
Ionic compounds do not exist as separate molecules. They exist as formula units in a lattice.
Formula Unit Mass Questions Class 9 Science Chapter 9
Q1. Calculate the formula unit mass of NaCl. Atomic masses: Na = 23u, Cl = 35.5u.
Ans.
Formula unit mass of NaCl
= 23 + 35.5
= 58.5u
Q2. Calculate the formula unit mass of CaCO₃. Atomic masses: Ca = 40u, C = 12u, O = 16u.
Ans.
Formula unit mass of CaCO₃
= 40 + 12 + 3(16)
= 40 + 12 + 48
= 100u
Q3. Calculate the formula unit mass of MgCl₂. Atomic masses: Mg = 24u, Cl = 35.5u.
Ans.
Formula unit mass of MgCl₂
= 24 + 2(35.5)
= 24 + 71
= 95u
Q4. Calculate the formula unit mass of Al₂O₃. Atomic masses: Al = 27u, O = 16u.
Ans.
Formula unit mass of Al₂O₃
= 2(27) + 3(16)
= 54 + 48
= 102u
Important Questions on Laws of Chemical Combination Class 9
The two main laws are the Law of Conservation of Mass and the Law of Definite Proportions.
These laws form the base of Dalton’s atomic theory.
Law of Conservation of Mass Important Questions Class 9
This law is often tested through simple numericals.
The total mass before and after a reaction must remain the same.
Q1. In a reaction, 5.4 g of aluminium reacts with oxygen to form 10.2 g of aluminium oxide. How much oxygen reacted?
Ans.
Mass of aluminium + Mass of oxygen = Mass of aluminium oxide
5.4 + Mass of oxygen = 10.2
Mass of oxygen = 10.2 - 5.4
= 4.8 g
Q2. Who proposed the Law of Conservation of Mass?
Ans. Antoine Lavoisier proposed the Law of Conservation of Mass.
He showed that mass remains conserved in chemical reactions.
Q3. 3.6 g of water decomposes into hydrogen and oxygen. If 0.4 g hydrogen is produced, how much oxygen is produced?
Ans.
Mass of water = Mass of hydrogen + Mass of oxygen
3.6 = 0.4 + Mass of oxygen
Mass of oxygen = 3.6 - 0.4
= 3.2 g
Law of Definite Proportions Questions Class 9
This law explains fixed mass ratios in compounds.
It helps prove that a compound has a definite composition.
Law of Definite Proportions Questions
Q1. Two samples of CO₂ are collected from different sources. Sample A has 3 g carbon and 8 g oxygen. Sample B has 6 g carbon and 16 g oxygen. Show that they follow the Law of Definite Proportions.
Ans.
Sample A ratio:
C : O = 3 : 8
Sample B ratio:
C : O = 6 : 16
= 3 : 8
Both samples have the same carbon to oxygen mass ratio.
So, they follow the Law of Definite Proportions.
Q2. Who proposed the Law of Definite Proportions?
Ans. Joseph Proust proposed the Law of Definite Proportions.
It states that a compound always contains the same elements in a fixed mass ratio.
Dalton Atomic Theory Questions Class 9
Dalton atomic theory questions class 9 appear as short answers, long answers, and assertion-reason questions.
Do not only memorise the postulates. Learn which postulate explains each law.
Postulates of Dalton’s Atomic Theory Questions Class 9
Dalton’s theory connects atoms with fixed ratios and conservation of mass.
It gives a particle-based explanation of chemical combination.
Q1. List all postulates of Dalton’s atomic theory.
Ans.
- All matter is made of atoms.
- Atoms are indivisible in chemical reactions.
- Atoms of the same element are identical in mass and properties.
- Atoms of different elements have different masses and properties.
- Atoms combine in simple whole-number ratios to form compounds.
- Atoms are neither created nor destroyed in chemical reactions.
Q2. Which postulate of Dalton’s theory explains the Law of Conservation of Mass?
Ans. The postulate that atoms are neither created nor destroyed explains the Law of Conservation of Mass.
If atoms remain conserved, total mass also remains conserved.
Q3. How does Dalton’s atomic theory explain the Law of Definite Proportions?
Ans. Dalton stated that atoms of elements have fixed masses.
He also stated that atoms combine in simple whole-number ratios.
So, a compound always has the same elements in the same mass ratio.
Application-Based Questions on Atomic Theory Class 9
Application questions test whether students can apply Dalton’s ideas to modern Chemistry.
Use both the original theory and its limitations.
Dalton Atomic Theory Application Questions
Q1. Dalton said atoms are indivisible. This is now known to be incorrect. Does this make the whole theory useless?
Ans. No, it does not make the whole theory useless.
Dalton’s theory correctly explained the laws of chemical combination and compound formation.
Later discoveries of electrons, protons, neutrons, isotopes, and isobars modified some postulates.
The theory remains important as a foundation of atomic Chemistry.
Q2. Why are CO and CO₂ different compounds even though both contain carbon and oxygen?
Ans. CO and CO₂ have different carbon to oxygen ratios.
In CO, one carbon atom combines with one oxygen atom.
In CO₂, one carbon atom combines with two oxygen atoms.
Because the fixed combining ratio is different, they are different compounds.
Molecules and Chemical Bonds Important Questions Class 9
Atoms combine to achieve stability.
They may share electrons or transfer electrons depending on the type of atoms involved.
Covalent Bond Questions Class 9 Science
Covalent bonds usually form between non-metal atoms.
The atoms share electrons to complete their outer shells.
Q1. What is a covalent bond? Give one example.
Ans. A covalent bond forms when two atoms share one or more pairs of electrons.
Example: In water, H₂O, each hydrogen atom shares one electron with oxygen.
Q2. Explain the structure of a nitrogen molecule, N₂. Identify the bond type.
Ans. A nitrogen molecule contains two nitrogen atoms.
Each nitrogen atom shares three electrons with the other nitrogen atom.
This forms a triple covalent bond, written as N≡N.
Ionic Bond Questions Class 9 Science
Ionic bonds form by electron transfer.
They form between oppositely charged ions.
Q1. What is an ionic bond? How does it form in sodium chloride?
Ans. An ionic bond forms when one atom transfers electrons to another atom.
In sodium chloride, sodium loses one electron and becomes Na⁺.
Chlorine gains one electron and becomes Cl⁻.
The attraction between Na⁺ and Cl⁻ forms the ionic bond.
Q2. Why do ionic compounds have high melting points?
Ans. Ionic compounds have strong electrostatic forces between oppositely charged ions.
A large amount of heat energy is needed to break these forces.
So, ionic compounds usually have high melting points.
Chemical Formula Questions Class 9 Science Chapter 9
Chemical formula questions class 9 test valency and the criss-cross method.
Do not memorise every formula. Learn how to derive it.
Writing Chemical Formula Questions Class 9
Write symbols first, then valencies, then criss-cross.
Use brackets when a polyatomic ion appears more than once.
Chemical Formula Questions Class 9
Q1. Write the chemical formulae for magnesium nitride, calcium phosphate, and ammonium sulphate.
Ans.
Magnesium nitride:
Mg has valency 2. Nitride has valency 3.
Formula = Mg₃N₂
Calcium phosphate:
Ca has valency 2. Phosphate, PO₄, has valency 3.
Formula = Ca₃(PO₄)₂
Ammonium sulphate:
Ammonium, NH₄, has valency 1. Sulphate, SO₄, has valency 2.
Formula = (NH₄)₂SO₄
Q2. Write the chemical formula for aluminium sulphate.
Ans.
Aluminium has valency 3.
Sulphate has valency 2.
Using the criss-cross method:
Al₂(SO₄)₃
So, the formula is Al₂(SO₄)₃.
Q3. Write the chemical formula for calcium hydroxide.
Ans.
Calcium has valency 2.
Hydroxide, OH, has valency 1.
Using the criss-cross method:
Ca(OH)₂
So, the formula is Ca(OH)₂.
Valency-Based Questions Class 9 Science
Valency is the combining capacity of an element or ion.
It helps decide the correct chemical formula.
Valency Questions Class 9
Q1. What is valency? How does it determine a chemical formula?
Ans. Valency is the combining capacity of an element.
In the criss-cross method, the valency of one element becomes the subscript of the other.
Example:
Al has valency 3. O has valency 2.
Formula = Al₂O₃
Q2. Write the valencies of sodium, calcium, ferric iron, and aluminium.
Ans.
| Element / Ion |
Valency |
| Sodium, Na |
1 |
| Calcium, Ca |
2 |
| Ferric iron, Fe³⁺ |
3 |
| Aluminium, Al |
3 |
Q3. Why are brackets used in Ca(OH)₂ but not in CaO?
Ans. Brackets are used when a polyatomic ion appears more than once.
In Ca(OH)₂, the hydroxide ion, OH⁻, appears twice.
In CaO, oxygen is a single atom ion, so brackets are not needed.
Class 9 Science Chapter 9 Assertion Reason Questions
Assertion-reason questions test whether the reason explains the statement correctly.
Read both statements before choosing the answer.
Directions:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Class 9 Science Chapter 9 Assertion Reason Questions
Q1. Assertion (A): The Law of Conservation of Mass holds true for chemical reactions. Reason (R): Atoms are neither created nor destroyed in a chemical reaction.
Ans. (a)
Both A and R are true, and R correctly explains A.
Mass remains conserved because atoms remain conserved.
Q2. Assertion (A): CO and CO₂ are different compounds even though both contain carbon and oxygen. Reason (R): The ratio of carbon to oxygen differs in CO and CO₂.
Ans. (a)
Both A and R are true, and R correctly explains A.
Different fixed ratios produce different compounds.
Q3. Assertion (A): Dalton’s atomic theory is completely correct. Reason (R): Discovery of isotopes and subatomic particles modified some postulates of Dalton’s theory.
Ans. (d)
A is false, but R is true.
Dalton’s theory was modified after later discoveries.
Q4. Assertion (A): Molecular mass and formula unit mass are calculated by adding atomic masses. Reason (R): Both use the chemical formula to count atoms or ions present in the substance.
Ans. (a)
Both A and R are true, and R correctly explains A.
Q5. Assertion (A): NaCl is written as a formula unit, not as a molecule. Reason (R): Ionic compounds exist as lattices, not as separate molecules.
Ans. (a)
Both A and R are true, and R correctly explains A.
Case Study Questions Class 9 Science Chapter 9
Case study questions connect laws, formulae, and numericals.
Read the values carefully and identify the law or formula being tested.
Case Study 1: Formation of Water
Meera takes 4 g of hydrogen and 32 g of oxygen in a closed vessel. After the reaction, 36 g of water forms.
Q1. Which law does this experiment demonstrate?
Ans. This experiment demonstrates the Law of Conservation of Mass.
Mass of reactants = 4 + 32 = 36 g
Mass of product = 36 g
So, mass is conserved.
Q2. If Meera uses 8 g of hydrogen and 64 g of oxygen, how much water forms?
Ans.
Mass of reactants = 8 + 64
= 72 g
So, 72 g of water forms.
Q3. What is the mass ratio of hydrogen to oxygen in water? Which law does this illustrate?
Ans.
Hydrogen : Oxygen = 4 : 32
= 1 : 8
This illustrates the Law of Definite Proportions.
Case Study 2: Aluminium Sulphate
Ravi is given a compound with formula Al₂(SO₄)₃.
Q1. Calculate the formula mass of Al₂(SO₄)₃. Atomic masses: Al = 27u, S = 32u, O = 16u.
Ans.
Formula mass of Al₂(SO₄)₃
= 2(27) + 3(32) + 12(16)
= 54 + 96 + 192
= 342u
Q2. What are the valencies of aluminium and sulphate in this compound?
Ans. Aluminium has valency 3.
Sulphate has valency 2.
The criss-cross of 3 and 2 gives Al₂(SO₄)₃.
Q3. Why are brackets used around SO₄ in Al₂(SO₄)₃?
Ans. SO₄ is a polyatomic ion.
There are three sulphate ions in the formula.
So, brackets are used to show that the entire SO₄ group is repeated three times.
Important Definitions and Formulas Class 9 Science Chapter 9
| Term |
Definition / Formula |
| Atom |
Smallest particle of an element that takes part in a chemical reaction |
| Molecule |
Smallest particle of an element or compound that can exist independently |
| Valency |
Combining capacity of an element or ion |
| Atomicity |
Number of atoms in one molecule of an element |
| Molecular mass |
Sum of atomic masses of all atoms in a molecule |
| Formula unit mass |
Sum of atomic masses in one formula unit of an ionic compound |
| Molar mass |
Mass of one mole of a substance |
| Law of Conservation of Mass |
Total mass of reactants = total mass of products |
| Law of Definite Proportions |
A compound has fixed elements in fixed mass ratio |
| Molecular mass formula |
Sum of atomic mass × number of atoms |
| Formula unit mass formula |
Sum of atomic masses in the formula unit |