Class 9 Science Important Questions and Answers: Atomic Foundation of Matter
Premium NCERT, Board Exam, Exemplar and Olympiad-level question answers for Class 9 Science Atomic Foundation of Matter. Learn laws of chemical combination, Dalton’s atomic theory, atoms, molecules, chemical formulae, ions, compounds, ionic and covalent compounds, atomic mass unit, molecular mass, formula unit mass and average atomic mass.
Short Introduction
Atomic Foundation of Matter explains how matter is made up of extremely small particles called atoms. It also explains how atoms combine to form molecules, ions and compounds.
This chapter connects basic chemistry with laws of chemical combination, Dalton’s atomic theory, chemical formulae and calculations of molecular mass and formula unit mass.
Scoring area: Laws of chemical combination, Dalton’s theory, writing chemical formulae, valency-based formula writing, ions, polyatomic ions and molecular mass calculations are very important for CBSE exams.
Chapter Overview
1. Laws of Chemical Combination
Elements combine according to fixed rules such as conservation of mass and constant proportions.
2. Dalton’s Atomic Theory
Dalton explained matter in terms of atoms and their combination in fixed ratios.
3. Atoms and Molecules
Atoms are the smallest particles of elements. Molecules are groups of atoms chemically bonded together.
4. Compounds
Compounds are pure substances formed when two or more elements combine chemically in a fixed ratio.
5. Ions and Formulae
Ions are charged particles. Chemical formulae show the kinds and numbers of atoms or ions in a compound.
6. Mass Calculations
Atomic mass, molecular mass, formula unit mass and average atomic mass are calculated using atomic masses.
Important Keywords
Important Rules and Formulae
Law of Conservation of Mass
Mass of reactants = Mass of products
Law of Constant Proportions
A compound always contains the same elements in a fixed ratio by mass.
Atomic Mass Unit
1 u = 1/12 of the mass of one carbon-12 atom
Molecular Mass
Sum of atomic masses of all atoms in a molecule
Formula Unit Mass
Sum of atomic masses of all ions in one formula unit
Average Atomic Mass
Weighted average of masses of naturally occurring isotopes
Cation
Positively charged ion
Anion
Negatively charged ion
Important note: In chemical formulae, the charge or valency of one ion becomes the subscript of the other ion after simplification. Example: Mg2+ and Cl– form MgCl2.
Common Ions and Polyatomic Ions
| Ion Name | Formula | Charge |
|---|---|---|
| Sodium ion | Na+ | +1 |
| Magnesium ion | Mg2+ | +2 |
| Aluminium ion | Al3+ | +3 |
| Chloride ion | Cl– | -1 |
| Oxide ion | O2- | -2 |
| Hydroxide ion | OH– | -1 |
| Nitrate ion | NO3– | -1 |
| Sulphate ion | SO42- | -2 |
| Carbonate ion | CO32- | -2 |
| Ammonium ion | NH4+ | +1 |
Important Very Short Answer Questions
Q1. What is an atom?
Answer: An atom is the smallest particle of an element that can take part in a chemical reaction.
Q2. What is a molecule?
Answer: A molecule is a group of two or more atoms chemically bonded together.
Q3. State the law of conservation of mass.
Answer: The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction.
Q4. State the law of constant proportions.
Answer: The law of constant proportions states that a pure chemical compound always contains the same elements combined together in a fixed ratio by mass.
Q5. Who proposed the atomic theory?
Answer: John Dalton proposed the atomic theory.
Q6. What is a chemical reaction?
Answer: A chemical reaction is a process in which reactants change into new substances called products.
Q7. What is a compound?
Answer: A compound is a pure substance formed when two or more elements combine chemically in a fixed ratio.
Q8. What is an ion?
Answer: An ion is an atom or group of atoms carrying a positive or negative charge.
Q9. What is a cation?
Answer: A positively charged ion is called a cation. Example: Na+.
Q10. What is an anion?
Answer: A negatively charged ion is called an anion. Example: Cl–.
Q11. What is a polyatomic ion?
Answer: A polyatomic ion is a group of atoms carrying a charge. Example: SO42-.
Q12. What is atomic mass unit?
Answer: Atomic mass unit is one-twelfth of the mass of one carbon-12 atom. It is written as u.
Q13. What is molecular mass?
Answer: Molecular mass is the sum of atomic masses of all atoms present in one molecule.
Q14. What is formula unit mass?
Answer: Formula unit mass is the sum of atomic masses of all ions present in one formula unit of an ionic compound.
Short Answer Questions
Q1. Differentiate between atom and molecule.
Answer:
| Atom | Molecule |
|---|---|
| It is the smallest particle of an element that can take part in a chemical reaction. | It is a group of two or more atoms chemically bonded together. |
| It may or may not exist independently. | It can usually exist independently. |
| Example: H, O, Na. | Example: H2, O2, H2O. |
Q2. Differentiate between molecule of an element and molecule of a compound.
Answer:
| Molecule of an Element | Molecule of a Compound |
|---|---|
| It contains atoms of the same element. | It contains atoms of different elements. |
| Example: O2, N2, Cl2. | Example: H2O, CO2, NH3. |
| It represents one element. | It represents a compound. |
Q3. Write the main points of Dalton’s atomic theory.
Answer:
- All matter is made up of tiny particles called atoms.
- Atoms are indivisible particles in a chemical reaction.
- Atoms of the same element are identical in mass and chemical properties.
- Atoms of different elements have different masses and chemical properties.
- Atoms combine in small whole-number ratios to form compounds.
- The relative number and kinds of atoms are constant in a given compound.
Q4. How did Dalton’s atomic theory explain the law of conservation of mass?
Answer: According to Dalton’s theory, atoms can neither be created nor destroyed in a chemical reaction. They are only rearranged. Therefore, the total mass of reactants remains equal to the total mass of products.
Q5. How did Dalton’s atomic theory explain the law of constant proportions?
Answer: According to Dalton’s theory, atoms combine in fixed whole-number ratios to form compounds. Therefore, a compound always has the same elements in a fixed ratio by mass.
Q6. Differentiate between ionic and covalent compounds.
Answer:
| Ionic Compounds | Covalent Compounds |
|---|---|
| They are formed by transfer of electrons. | They are formed by sharing of electrons. |
| They contain ions. | They contain molecules. |
| Usually formed between metals and non-metals. | Usually formed between non-metals. |
| Example: NaCl, MgO. | Example: H2O, CO2. |
Q7. What are chemical formulae? Why are they useful?
Answer: A chemical formula is a symbolic representation of a molecule or compound.
- It shows the elements present in a substance.
- It shows the number of atoms of each element.
- It helps in writing chemical reactions.
- It helps in calculating molecular mass or formula unit mass.
Q8. What is the difference between molecular mass and formula unit mass?
Answer:
| Molecular Mass | Formula Unit Mass |
|---|---|
| It is used for molecules. | It is used for ionic compounds. |
| It is the sum of atomic masses of all atoms in a molecule. | It is the sum of atomic masses of all ions in one formula unit. |
| Example: H2O, CO2. | Example: NaCl, CaCl2. |
Q9. What is average atomic mass?
Answer: Average atomic mass is the weighted average of the masses of all naturally occurring isotopes of an element, according to their relative abundance.
Q10. Why is atomic mass expressed in atomic mass unit?
Answer: Atoms are extremely small and their actual masses are very difficult to express in grams. Therefore, atomic masses are expressed in atomic mass unit, written as u.
Long Answer Questions
Q1. Explain the two laws of chemical combination with examples.
Answer:
1. Law of Conservation of Mass:
- It states that mass can neither be created nor destroyed in a chemical reaction.
- Total mass of reactants is equal to total mass of products.
- Example: If 12 g carbon reacts with 32 g oxygen, 44 g carbon dioxide is formed.
- Mass of reactants = 12 g + 32 g = 44 g.
- Mass of product = 44 g.
2. Law of Constant Proportions:
- It states that a pure compound always contains the same elements in a fixed ratio by mass.
- Example: Water always contains hydrogen and oxygen in the ratio 1:8 by mass.
- This ratio remains the same regardless of the source of water.
Q2. Explain Dalton’s atomic theory and mention its importance.
Answer: Dalton’s atomic theory was an important theory that explained the nature of matter and chemical combination.
- All matter is made up of atoms.
- Atoms are very small and indivisible particles in chemical reactions.
- Atoms of the same element have the same mass and chemical properties.
- Atoms of different elements have different masses and chemical properties.
- Atoms combine in simple whole-number ratios to form compounds.
- In a given compound, the relative number and kinds of atoms are fixed.
Importance: Dalton’s theory explained the law of conservation of mass and law of constant proportions. It also gave a scientific basis to the study of atoms and molecules.
Q3. Explain how to write the chemical formula of an ionic compound using valency.
Answer:
- Write the symbols of the positive and negative ions.
- Write their valencies or charges.
- Criss-cross the valencies to get the subscripts.
- Simplify the ratio if possible.
- Do not write subscript 1.
Example: Formula of aluminium oxide
- Aluminium ion = Al3+
- Oxide ion = O2-
- Criss-cross the charges: Al2O3
- Formula of aluminium oxide = Al2O3
Q4. Explain ions, cations, anions and polyatomic ions with examples.
Answer:
- Ion: An atom or group of atoms having a positive or negative charge is called an ion.
- Cation: A positively charged ion is called a cation. Example: Na+, Ca2+.
- Anion: A negatively charged ion is called an anion. Example: Cl–, O2-.
- Polyatomic ion: A group of atoms carrying a charge is called a polyatomic ion. Example: NH4+, SO42-, NO3–.
Q5. Explain molecular mass and formula unit mass with examples.
Answer:
Molecular mass: It is the sum of atomic masses of all atoms present in one molecule.
Example: Molecular mass of H2O = 2 × mass of H + 1 × mass of O = 2 × 1 + 16 = 18 u.
Formula unit mass: It is the sum of atomic masses of all ions present in one formula unit of an ionic compound.
Example: Formula unit mass of NaCl = mass of Na + mass of Cl = 23 + 35.5 = 58.5 u.
Important Numericals with Solutions
Numerical 1: Calculate the molecular mass of H2O.
Solution:
Atomic mass of H = 1 u
Atomic mass of O = 16 u
Molecular mass of H2O = 2 × 1 + 16
= 2 + 16 = 18 u
Numerical 2: Calculate the molecular mass of CO2.
Solution:
Atomic mass of C = 12 u
Atomic mass of O = 16 u
Molecular mass of CO2 = 12 + 2 × 16
= 12 + 32 = 44 u
Numerical 3: Calculate the formula unit mass of NaCl.
Solution:
Atomic mass of Na = 23 u
Atomic mass of Cl = 35.5 u
Formula unit mass of NaCl = 23 + 35.5
= 58.5 u
Numerical 4: Calculate the molecular mass of H2SO4.
Solution:
Atomic mass of H = 1 u, S = 32 u, O = 16 u
Molecular mass of H2SO4 = 2 × 1 + 32 + 4 × 16
= 2 + 32 + 64 = 98 u
Numerical 5: Calculate the formula unit mass of CaCO3.
Solution:
Atomic mass of Ca = 40 u, C = 12 u, O = 16 u
Formula unit mass of CaCO3 = 40 + 12 + 3 × 16
= 40 + 12 + 48 = 100 u
Numerical 6: A compound contains hydrogen and oxygen in the ratio 1:8 by mass. If 3 g of hydrogen reacts completely, how much oxygen is required?
Solution:
Ratio of hydrogen : oxygen = 1 : 8
1 g hydrogen requires 8 g oxygen
3 g hydrogen requires 3 × 8 = 24 g oxygen
Numerical 7: In a reaction, 5 g of calcium carbonate gives 2.8 g of calcium oxide and 2.2 g of carbon dioxide. Verify the law of conservation of mass.
Solution:
Mass of reactant = 5 g
Mass of products = 2.8 g + 2.2 g = 5 g
Mass of reactant = Mass of products
Hence, the law of conservation of mass is verified.
Numerical 8: Calculate the average atomic mass of chlorine if chlorine has two isotopes, Cl-35 and Cl-37, in the ratio 3:1.
Solution:
Average atomic mass = [(35 × 3) + (37 × 1)] ÷ 4
= (105 + 37) ÷ 4
= 142 ÷ 4 = 35.5 u
Numerical 9: Write the chemical formula of calcium chloride.
Solution:
Calcium ion = Ca2+
Chloride ion = Cl–
Criss-cross the charges.
Formula = CaCl2
Numerical 10: Write the chemical formula of aluminium sulphate.
Solution:
Aluminium ion = Al3+
Sulphate ion = SO42-
Criss-cross the charges.
Formula = Al2(SO4)3
Case-Study Based Questions
Case Study 1: Law of Conservation of Mass
A student heats 10 g of a substance in a closed container. After the reaction, two products are formed. Product A has mass 6 g and Product B has mass 4 g.
Q1. What is the total mass of products?
Answer: Total mass of products = 6 g + 4 g = 10 g.
Q2. Which law is verified in this case?
Answer: The law of conservation of mass is verified.
Q3. State the law involved.
Answer: Mass can neither be created nor destroyed in a chemical reaction.
Q4. Why is the container closed?
Answer: The container is closed so that no reactant or product can escape during the reaction.
Case Study 2: Chemical Formula Writing
A compound is formed by the combination of magnesium ions and nitrate ions. Magnesium forms Mg2+, while nitrate is a polyatomic ion represented as NO3–.
Q1. Identify the cation.
Answer: Mg2+ is the cation.
Q2. Identify the anion.
Answer: NO3– is the anion.
Q3. Write the chemical formula of magnesium nitrate.
Answer: The chemical formula is Mg(NO3)2.
Q4. Why are brackets used in the formula?
Answer: Brackets are used because nitrate is a polyatomic ion and two nitrate ions are required.
Case Study 3: Average Atomic Mass
An element X has two isotopes. One isotope has mass 10 u and abundance 20%. The second isotope has mass 11 u and abundance 80%.
Q1. What is meant by average atomic mass?
Answer: It is the weighted average of the masses of naturally occurring isotopes of an element.
Q2. Calculate the average atomic mass of element X.
Answer: Average atomic mass = (10 × 20 + 11 × 80) ÷ 100 = (200 + 880) ÷ 100 = 10.8 u.
Q3. Why is average atomic mass not always a whole number?
Answer: It is not always a whole number because it is calculated as a weighted average of isotopes.
Q4. What is the unit of atomic mass?
Answer: The unit of atomic mass is atomic mass unit, written as u.
Assertion-Reason Questions
Choose the correct option:
A. Both Assertion and Reason are true and Reason is the correct explanation of Assertion.
B. Both Assertion and Reason are true but Reason is not the correct explanation of Assertion.
C. Assertion is true but Reason is false.
D. Assertion is false but Reason is true.
Q1. Assertion: Mass remains conserved in a chemical reaction.
Reason: Atoms are neither created nor destroyed during a chemical reaction.
Answer: A. Both are true and Reason correctly explains Assertion.
Q2. Assertion: Water always contains hydrogen and oxygen in the ratio 1:8 by mass.
Reason: A pure compound always contains elements in a fixed ratio by mass.
Answer: A. Both are true and Reason correctly explains Assertion.
Q3. Assertion: Na+ is an anion.
Reason: Anions are negatively charged ions.
Answer: D. Assertion is false but Reason is true.
Q4. Assertion: SO42- is a polyatomic ion.
Reason: A polyatomic ion is a group of atoms carrying a charge.
Answer: A. Both are true and Reason correctly explains Assertion.
Q5. Assertion: Ionic compounds are represented by formula unit mass.
Reason: Ionic compounds contain ions arranged in a fixed ratio, not separate molecules.
Answer: A. Both are true and Reason correctly explains Assertion.
Q6. Assertion: Molecular mass of CO2 is 28 u.
Reason: Molecular mass is the sum of atomic masses of all atoms in a molecule.
Answer: D. Assertion is false but Reason is true. The molecular mass of CO2 is 44 u.
Exam Tips
- Write laws of chemical combination in exact NCERT-style language.
- Remember that conservation of mass works best when the reaction occurs in a closed system.
- For Dalton’s theory, write points in short bullet form.
- Learn common ions and polyatomic ions because they are essential for formula writing.
- Use the criss-cross method carefully while writing chemical formulae.
- Always simplify subscripts if required.
- Use brackets when more than one polyatomic ion is present in a formula.
- For molecular mass numericals, multiply atomic mass by the number of atoms shown in the formula.
- Use formula unit mass for ionic compounds like NaCl, CaCl2 and Al2O3.
- Do not confuse atomic mass, molecular mass and average atomic mass.
Quick Revision Box
Atom
Smallest particle of an element that takes part in a chemical reaction.
Molecule
Group of two or more atoms chemically bonded together.
Compound
Pure substance formed by chemical combination of elements in a fixed ratio.
Conservation of Mass
Mass can neither be created nor destroyed in a chemical reaction.
Constant Proportions
A compound always contains elements in a fixed ratio by mass.
Dalton’s Theory
Matter is made up of atoms that combine in simple whole-number ratios.
Cation
Positively charged ion.
Anion
Negatively charged ion.
Polyatomic Ion
Group of atoms carrying a charge.
Molecular Mass
Sum of atomic masses of all atoms in a molecule.
Formula Unit Mass
Sum of atomic masses of all ions in one formula unit.
Average Atomic Mass
Weighted average of masses of naturally occurring isotopes.
FAQ Section
What is the main focus of Atomic Foundation of Matter?
The main focus is to understand atoms, molecules, laws of chemical combination, Dalton’s atomic theory, ions, compounds, chemical formulae and mass calculations.
What are the two important laws of chemical combination?
The two important laws are the law of conservation of mass and the law of constant proportions.
Who gave the atomic theory?
John Dalton gave the atomic theory.
What is the difference between atom and molecule?
An atom is the smallest particle of an element, while a molecule is a group of atoms chemically bonded together.
What is a chemical formula?
A chemical formula is a symbolic representation that shows the elements present and the number of atoms or ions in a substance.
What is the difference between ionic and covalent compounds?
Ionic compounds are formed by transfer of electrons, while covalent compounds are formed by sharing of electrons.
What is a polyatomic ion?
A polyatomic ion is a group of atoms carrying a positive or negative charge.
What is atomic mass unit?
Atomic mass unit is one-twelfth of the mass of one carbon-12 atom. It is written as u.
What is molecular mass?
Molecular mass is the sum of atomic masses of all atoms present in one molecule.
What is formula unit mass?
Formula unit mass is the sum of atomic masses of all ions present in one formula unit of an ionic compound.
What is average atomic mass?
Average atomic mass is the weighted average of the masses of naturally occurring isotopes of an element.
Final Conclusion
Atomic Foundation of Matter is a key chapter that builds the base for chemistry in higher classes. It explains how atoms combine to form molecules, ions and compounds and how chemical formulae are written.
To score well, students should practise laws of chemical combination, Dalton’s atomic theory, common ions, formula writing and molecular mass numericals. Clear understanding of these topics will help in NCERT questions, CBSE board exams, Exemplar questions and Olympiad-level reasoning.
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