Class 9 Science Chapter 9 | Atomic Foundations of Matter

Chapter Overview

Chapter 8 introduced the structure of the atom, subatomic particles and valence electrons. Chapter 9 builds on that foundation to explain how atoms combine, why compounds have definite composition, how chemical formulae are written, and how their masses are calculated.

Core idea
Atoms combine in definite ways to form molecules and compounds.

Two important laws
Conservation of Mass and Constant Proportions.

Bonding
Covalent bonding involves sharing; ionic bonding involves electron transfer and attraction between ions.

Exam skill
Write formulae and calculate molecular mass or formula-unit mass.

1. Why Do Atoms Combine?

Atoms combine because a more stable electronic arrangement can be achieved when their valence electrons are shared or transferred. The previous chapter introduced the idea of a stable valence shell; this chapter applies that idea to the formation of compounds.

Atoms

→

Valence electrons interact

→

Bond formation

→

Molecules / ionic compounds

Remember: A compound can have properties very different from those of the elements from which it forms. For example, hydrogen and oxygen are gases, while water is a liquid under ordinary conditions.

2. Law of Conservation of Mass

The Law of Conservation of Mass states that in a chemical reaction, mass is neither created nor destroyed. Therefore, the total mass of the reactants is equal to the total mass of the products.

Total mass of reactants = Total mass of products

This law is associated with Antoine Lavoisier. In experiments, an apparent loss of mass can occur if a gaseous product escapes from an open container. If the system is closed and the gas is retained, the total mass remains constant.

Simple example

If 12 g of carbon reacts completely with 32 g of oxygen to form carbon dioxide, the mass of carbon dioxide formed is 44 g.

Reactants	Total mass	Product	Total mass
12 g carbon + 32 g oxygen	44 g	Carbon dioxide	44 g

3. Law of Constant Proportions

The Law of Constant Proportions states that a pure chemical compound always contains the same elements combined in the same fixed ratio by mass, irrespective of the source of the compound.

This law is associated with Joseph Proust.

Water example: Hydrogen and oxygen in pure water are present in a fixed mass ratio of 1 : 8.

Thus, 9 g of water contains 1 g hydrogen and 8 g oxygen. A larger quantity of pure water preserves the same mass ratio.

Compound vs mixture

Compound	Mixture
Elements are chemically combined.	Substances are physically mixed.
Has a fixed composition.	Composition can vary.
Shows properties different from constituent elements.	Constituents generally retain their individual properties.

4. Dalton’s Atomic Theory

John Dalton developed an atomic theory that helped explain the laws of chemical combination.

Matter is composed of very small particles called atoms.
Atoms of the same element were considered identical in their mass and properties in Dalton's model.
Atoms of different elements have different properties and masses.
Atoms combine in simple whole-number ratios to form compounds.
Atoms are rearranged during chemical reactions; they are not created or destroyed in the reaction.

Modern note: Some details of Dalton's original theory were later modified by the discovery of subatomic particles, isotopes and nuclear processes. For Class 9, focus on how the theory explains chemical combination.

5. Atoms, Molecules and Chemical Bonds

Atoms

An atom is the basic particle of an element that participates in chemical combination.

Molecules

A molecule is a group of two or more atoms held together by chemical bonds and capable of existing as an independent unit in molecular substances.

Chemical bond

A chemical bond is an attractive interaction that holds atoms or ions together. At the Class 9 level, the chapter focuses mainly on covalent and ionic bonding.

6. Covalent Bonding — Sharing of Electrons

A covalent bond forms when atoms share one or more pairs of electrons. This type of bonding is commonly found between non-metal atoms.

Examples

Substance	Representation	Key idea
Hydrogen	H—H	One shared pair
Oxygen	O=O	Two shared pairs
Nitrogen	N≡N	Three shared pairs
Water	H—O—H	Oxygen shares electrons with two hydrogen atoms

Atoms approach

→

Valence electrons are shared

→

Shared pair forms bond

→

Stable arrangement

7. Ionic Bonding — Transfer of Electrons

An ionic bond results when electrons are transferred from one atom to another, producing oppositely charged ions that attract each other electrostatically.

Sodium chloride (NaCl)

Na loses 1 e⁻

→

Na⁺

+

Cl gains 1 e⁻

→

Cl⁻

→

NaCl

Cation: positively charged ion, usually formed by loss of electrons.

Anion: negatively charged ion, usually formed by gain of electrons.

Important: The ionic compound is electrically neutral overall because the total positive charge balances the total negative charge.

8. Ionic vs Covalent Compounds

Feature	Ionic compounds	Covalent compounds
Formation	Electron transfer followed by attraction between ions	Sharing of electron pairs
Typical constituents	Often metal + non-metal	Often non-metal + non-metal
Particles	Ions arranged in a lattice	Molecules in many molecular substances
Melting/boiling points	Generally high	Often relatively low for molecular substances
Electrical conduction	Conduct when molten or dissolved because ions can move	Generally do not conduct electricity
Examples	NaCl, MgO, CaCl₂	H₂O, CO₂, CH₄

9. Writing Chemical Formulae

A chemical formula tells us which elements are present and the number or simplest whole-number ratio of their atoms/ions.

Valency and charge

Valency indicates the combining capacity of an atom or ion. For ionic compounds, the total positive and negative charge must balance.

Ion	Charge / valency
Na⁺	1
Mg²⁺	2
Al³⁺	3
Cl⁻	1
O²⁻	2
N³⁻	3

Examples

Compound	Charge balance	Formula
Sodium chloride	Na⁺ + Cl⁻	NaCl
Magnesium chloride	Mg²⁺ + 2Cl⁻	MgCl₂
Aluminium oxide	2Al³⁺ + 3O²⁻	Al₂O₃
Calcium chloride	Ca²⁺ + 2Cl⁻	CaCl₂

Criss-cross shortcut: The numerical values of the charges can be used as subscripts of the other ion, then simplify the ratio if necessary. Always check that the final formula is electrically neutral.

10. Naming and Reading Formulae

NaCl: sodium chloride
MgO: magnesium oxide
CaCl₂: calcium chloride
Al₂O₃: aluminium oxide
H₂O: water
CO₂: carbon dioxide
CH₄: methane

In an ionic formula, the subscripts usually show the simplest whole-number ratio of ions. For molecular compounds, the formula can show the actual number of atoms in one molecule.

11. Molecular Mass

Molecular mass is the sum of the atomic masses of all atoms present in one molecule. It is expressed in unified atomic mass units (u).

Molecular mass = Σ (number of atoms × atomic mass)

Example: H₂O

H = 1 u and O = 16 u

H₂O = (2 × 1) + (1 × 16) = 18 u

Example: CO₂

CO₂ = (1 × 12) + (2 × 16) = 44 u

Example: NH₃

NH₃ = (1 × 14) + (3 × 1) = 17 u

12. Formula Unit Mass

Ionic compounds do not consist of independent molecules in the same way molecular substances do. Their mass is therefore commonly expressed as formula unit mass, obtained by adding atomic masses according to the simplest formula unit.

Formula unit mass = Σ (atomic masses according to the formula unit)

Example: NaCl

NaCl = 23 + 35.5 = 58.5 u

Example: CaCl₂

CaCl₂ = 40 + (2 × 35.5) = 111 u

Example: K₂SO₄

K₂SO₄ = (2 × 39) + 32 + (4 × 16) = 174 u

13. Worked Numericals — Exam Practice

Q1. Find the molecular mass of H₂SO₄.

H = 1, S = 32, O = 16.

2(1) + 32 + 4(16) = 98 u

Q2. Find the formula unit mass of MgO.

24 + 16 = 40 u

Q3. Find the molecular mass of CO₂.

12 + 2(16) = 44 u

Q4. Why is the formula of magnesium chloride MgCl₂?

Mg forms Mg²⁺ and chlorine forms Cl⁻. Two chloride ions are needed to balance one Mg²⁺ ion, so the neutral formula is MgCl₂.

14. Common Mistakes to Avoid

Do not confuse atomic number with mass number from Chapter 8.
Do not change subscripts randomly while balancing charges.
Do not forget to simplify a ratio if the criss-cross method produces a common factor.
Do not call NaCl a molecule in the strict ionic-lattice sense; use formula unit.
Do not say solid ionic compounds conduct because their ions are fixed in place.
Always include the correct unit, such as u, in molecular/formula-unit mass questions.

15. Quick Revision Sheet

Concept	Remember
Conservation of Mass	Total mass of reactants = total mass of products.
Constant Proportions	A pure compound has a fixed ratio of elements by mass.
Lavoisier	Associated with conservation of mass.
Proust	Associated with constant proportions.
Dalton	Atomic theory; simple whole-number ratios.
Covalent bond	Sharing of electron pairs.
Ionic bond	Electron transfer + electrostatic attraction of ions.
Cation	Positive ion.
Anion	Negative ion.
Molecular mass	Sum of atomic masses in a molecule.
Formula unit mass	Sum of atomic masses in the simplest ionic formula unit.

16. Formula Box

Conservation
m_reactants = m_products

Molecular mass
Σ(number of atoms × atomic mass)

Formula-unit mass
Σ(atomic masses in formula unit)

Charge balance
Total positive charge = total negative charge

17. Practice Questions

State the Law of Conservation of Mass.
State the Law of Constant Proportions.
Why can an open reaction vessel appear to lose mass when a gas escapes?
How does Dalton's theory explain conservation of mass?
How does Dalton's theory explain fixed composition of compounds?
Define molecule and chemical bond.
What is a covalent bond?
What is an ionic bond?
Differentiate between cation and anion.
Explain the formation of NaCl using electron transfer.
Write the formula of magnesium oxide.
Write the formula of aluminium oxide.
Write the formula of calcium chloride.
Why is CaCl₂ electrically neutral?
Differentiate between ionic and covalent compounds.
Why do molten ionic compounds conduct electricity?
Why does solid NaCl not conduct electricity in the usual state?
Calculate the molecular mass of H₂O.
Calculate the molecular mass of CO₂.
Calculate the molecular mass of NH₃.
Calculate the formula unit mass of NaCl.
Calculate the formula unit mass of MgO.
Calculate the formula unit mass of CaCl₂.
Calculate the formula unit mass of K₂SO₄.
What does the formula H₂O tell us?
What is the difference between a molecular formula and an ionic formula unit?
Why are compounds different in properties from their constituent elements?
What is valency?
Explain why Al₂O₃ is the neutral formula for aluminium oxide.
Explain the difference between molecular mass and formula unit mass.

18. Interactive Quiz — 30 Questions

Select one option for each question and click Check My Score.

1. The Law of Conservation of Mass states that:

Mass is created during a reaction Mass is destroyed during a reaction Total mass of reactants equals total mass of products Only gases have mass

2. The Law of Constant Proportions says that elements in a pure compound combine:

In any ratio In a fixed ratio by mass Only in a 1:1 ratio Only by volume

3. Who proposed the Law of Conservation of Mass?

John Dalton Antoine Lavoisier J. J. Thomson Ernest Rutherford

4. The Law of Constant Proportions is associated with:

Proust Bohr Newton Chadwick

5. Dalton proposed his atomic theory in:

1789 1808 1897 1911

6. According to Dalton's theory, atoms of the same element were considered:

Identical in their properties Always charged Larger than molecules Unable to combine

7. Atoms combine to form compounds in:

Random ratios only Simple whole-number ratios Only decimal ratios Only equal masses

8. In a chemical reaction, atoms are mainly:

Created from nothing Destroyed completely Rearranged Changed into light

9. A molecule is best described as:

A single electron A group of atoms held together by chemical bonds Only a positively charged ion A nucleus

10. A covalent bond involves:

Transfer of protons Sharing of electrons Sharing of neutrons Transfer of nuclei

11. An ionic bond is formed mainly through:

Transfer of electrons and electrostatic attraction Sharing of protons Sharing of nuclei Loss of neutrons

12. When sodium loses one electron, it forms:

Na− Na+ Na2− Na2+

13. When chlorine gains one electron, it forms:

Cl+ Cl2+ Cl− Cl2−

14. A positively charged ion is called a:

Anion Cation Molecule Radical

15. A negatively charged ion is called an:

Anion Cation Atom Proton

16. The stable valence-shell arrangement commonly discussed in this chapter is the:

Octet Duet only Decet Triplet

17. The formula of sodium chloride is:

NaCl Na2Cl NaCl2 Na2Cl2

18. The formula of magnesium oxide is:

MgO Mg2O MgO2 Mg2O2

19. The formula of calcium chloride is:

CaCl CaCl2 Ca2Cl Ca2Cl2

20. The formula of aluminium oxide is:

AlO AlO2 Al2O3 Al3O2

21. Which is a covalent molecule?

NaCl MgO H2O CaCl2

22. Which compound is typically ionic?

CO2 H2O NaCl CH4

23. Ionic compounds generally have:

High melting points Very low melting points No definite composition No ions

24. Ionic compounds conduct electricity when:

Always as solids In molten state or aqueous solution Only as crystals Only as gases

25. Covalent compounds generally:

Conduct electricity readily Do not conduct electricity in the usual state Contain only ions Have free electrons like metals

26. Molecular mass is found by:

Adding atomic masses of atoms in a molecule Subtracting proton masses Counting only electrons Dividing atomic mass by valency

27. The molecular mass of H2O is:

16 u 17 u 18 u 20 u

28. The formula unit mass of NaCl is approximately:

23 u 35.5 u 58.5 u 46 u

29. The simplest whole-number ratio represented by an ionic compound formula is called its:

Formula unit Nucleus Isotope Electron shell

30. Which pair is correctly matched?

Covalent—electron transfer Ionic—electron sharing only Cation—positive ion Anion—positive ion

19. Exam Focus

Learn both laws of chemical combination with their meaning and examples.
Understand Dalton's theory as an explanation of fixed composition and conservation of mass.
Practise ionic vs covalent bonding diagrams and explanations.
Memorise common ions and practise charge balancing.
Practise at least 5 molecular/formula-unit mass calculations.
In numerical answers, show the formula, substitution and final unit.