Science · Class 9 · Chapter 9

Atomic Foundations of Matter

• Law of Conservation of Mass — Mass is unchanged before and after a chemical reaction.
• Law of Constant Proportions — A compound's elements always combine in a fixed mass ratio.
• Molecule — A neutral group of atoms that exists independently.
• Chemical bond — The force holding atoms together in a molecule or crystal.
• Covalent bond — Formed by atoms sharing electrons.
• Ionic bond — Formed by atoms transferring electrons (forming ions).
• Molecular mass / Formula unit mass — Total mass of a covalent molecule / an ionic formula unit.
• Valency — Combining capacity — the charge or bonds an atom forms.

Chapter 9 · Concept 1 of 17

The Law of Conservation of Mass

• Vinegar and baking soda react in a sealed balloon: total mass before and after the fizzing reaction stays exactly the same.
• Proposed by Antoine Lavoisier, 1789 — Matter can neither be created nor destroyed in a chemical reaction — only rearranged. If mass seems to change, check whether a gas has escaped the system.

Chapter 9 · Concept 2 of 17

Checking the Law with Real Numbers

• Reactants | Products
• Calcium carbonate: 4.0 g — Carbon dioxide: 1.76 g
• Hydrochloric acid: 2.92 g — Water: 0.72 g
• Calcium chloride: 4.44 g
• Total = 6.92 g — Total = 6.92 g

Chapter 9 · Concept 3 of 17

The Law of Constant Proportions

• Water from a river, a borewell, or the ocean — once purified — always contains hydrogen and oxygen in the exact same mass ratio.
• Hydrogen : Oxygen = 1 : 8, always — Proposed by Joseph Proust. Decompose 9 g of pure water from any source, and you'll always get 1 g of hydrogen and 8 g of oxygen — regardless of where the water came from.

Chapter 9 · Concept 4 of 17

Using the Fixed Ratio to Predict Amounts

• Sodium chloride (NaCl) always contains sodium and chlorine in a mass ratio of 23 : 35.5.
• If 46 g of sodium reacts completely... — Chlorine needed = (35.5 ÷ 23) × 46 = 71 g. The ratio holds no matter the total quantity involved.

Chapter 9 · Concept 5 of 17

Dalton's Atomic Theory

• Postulate
• All matter is made of tiny particles called atoms.
• Atoms are indivisible — they cannot be created or destroyed in a reaction.
• Atoms of a given element are identical in mass and chemical properties.
• Atoms of different elements differ in mass and chemical properties.
• Atoms combine in simple whole-number ratios to form compounds.
• The relative number and kinds of atoms in a compound are constant.

Chapter 9 · Concept 6 of 17

How Atoms Combine

• Atoms combine to complete their outermost (valence) shell — the resulting arrangement has lower, more stable energy. This holding force is a chemical bond.
• Sharing electrons — Atoms share one or more pairs of valence electrons — a covalent bond.
• Transferring electrons — One atom gives up electrons, another accepts them — an ionic bond.

Chapter 9 · Concept 7 of 17

Covalent Bonds: Sharing to Complete a Shell

• Hydrogen (H—H) — Each H atom has 1 electron in the K-shell (needs 2). Two atoms share one electron each — a single covalent bond forms H₂.
• Chlorine (Cl—Cl) and Oxygen (O=O) — Cl needs 1 electron → single bond, Cl₂. Oxygen needs 2 electrons → shares two pairs, forming a double bond, O₂.

Chapter 9 · Concept 8 of 17

Covalent Bonds Between Different Elements

• Hydrogen chloride (H—Cl) — Both H and Cl need just 1 more electron — they share a single pair to form HCl, a covalent compound.
• Water (H₂O) — Oxygen needs 2 electrons, but each hydrogen offers only 1 — so two hydrogen atoms each share with the one oxygen atom, forming H₂O.

Chapter 9 · Concept 9 of 17

Naming Covalent Compounds

• Prefix | Meaning | Example
• mono- (usually dropped on the first element) — 1 — CO — carbon monoxide
• di- — 2 — CO₂ — carbon dioxide
• tri- — 3 — PCl₃ — phosphorus trichloride
• hexa- — 6 — SF₆ — sulfur hexafluoride

Chapter 9 · Concept 10 of 17

Ionic Bonds: Sodium Chloride

• Sodium loses an electron → Na⁺ — Sodium (2, 8, 1) loses its single valence electron, becoming a positively charged cation with 11 protons and 10 electrons.
• Chlorine gains an electron → Cl⁻ — Chlorine (2, 8, 7) gains that electron to complete its octet, becoming a negatively charged anion. Opposite charges attract — an ionic bond.

Chapter 9 · Concept 11 of 17

Ionic Compounds Form Crystal Lattices

• Ionic compounds don't exist as single molecules — ions pack into a repeating 3-D crystal structure.
• Every ion, surrounded — In sodium chloride, each Na⁺ ion is surrounded by six Cl⁻ ions, and each Cl⁻ by six Na⁺ ions — a regular, repeating pattern called a crystal lattice.

Chapter 9 · Concept 12 of 17

Some Common Ions

• Valency 1 | Valency 2 | Valency 3 | Polyatomic
• Na⁺, K⁺, Ag⁺, Cl⁻, F⁻ — Ca²⁺, Mg²⁺, Zn²⁺, O²⁻, S²⁻ — Al³⁺, Fe³⁺ — OH⁻, NO₃⁻, NH₄⁺
• CO₃²⁻, SO₄²⁻ (valency 2)

Chapter 9 · Concept 13 of 17

Writing Formulae: Covalent Compounds

• Write the symbols, write their valencies, then cross the valencies over as subscripts.
• Elements & valencies | Formula
• H (1) and Cl (1) — HCl
• H (1) and S (2) — H₂S
• C (4) and Cl (1) — CCl₄ — carbon tetrachloride

Chapter 9 · Concept 14 of 17

Writing Formulae: Ionic Compounds

• Cation first, then anion — cross the charge numbers, then simplify to the smallest whole-number ratio.
• Ions | Formula
• Ca²⁺ and Cl⁻ — CaCl₂
• Al³⁺ and O²⁻ — Al₂O₃
• Mg²⁺ and OH⁻ — Mg(OH)₂ — brackets group repeated polyatomic ions

Chapter 9 · Concept 15 of 17

Properties of Ionic and Covalent Compounds

• | Ionic compounds | Covalent compounds
• Solubility — Usually soluble in water — Usually soluble in kerosene, petrol
• Conducts electricity (solid) — No — ions are fixed in place — No
• Conducts electricity (dissolved/molten) — Yes — ions become free to move — Usually no
• Melting & boiling points — High (strong inter-ionic attraction) — Low

Chapter 9 · Concept 16 of 17

Molecular Mass

• Water (H₂O) — H = 1 u, O = 16 u. Molecular mass = (1 × 2) + (16 × 1) = 18 u.
• Carbon dioxide (CO₂) — C = 12 u, O = 16 u. Molecular mass = (12 × 1) + (16 × 2) = 44 u.

Chapter 9 · Concept 17 of 17

Formula Unit Mass

• Ionic compounds don't form molecules — so we add up the mass of one formula unit instead: the simplest whole-number ratio of ions.
• Sodium oxide (Na₂O) — Na = 23 u, O = 16 u. (23 × 2) + (16 × 1) = 62 u.
• Calcium nitrate, Ca(NO₃)₂ — Ca = 40 u, N = 14 u, O = 16 u. 40 + {(14 + 48)} × 2 = 164 u.

Quick Recap

Check Your Understanding

1. 1You want an ionic compound where total positive charge is 6+ and total negative charge is 6−. Which combination is correct? (i) 2 Al³⁺ and 3 Cl⁻ (ii) 3 Mg²⁺ and 1 PO₄³⁻ (iii) 2 Fe³⁺ and 3 O²⁻ (iv) 3 Ca²⁺ and 2 SO₄²⁻
2. 2Write the chemical formulae for: (i) Aluminium nitrate (ii) Calcium oxide (iii) Ferric oxide
3. 3Write the formulae of the compounds formed from: (i) Ca²⁺ and Br⁻ (ii) Al³⁺ and CO₃²⁻ (iii) K⁺ and SO₄²⁻ (iv) NH₄⁺ and Cl⁻