Chemistry — Equations, Reactions, Water, Atomic Theory & The Periodic Table
1. The Language of Chemistry
Symbols, Valency, and Formulae
- Each ELEMENT has a UNIQUE chemical symbol. First letter CAPITAL.
- Valency: Combining capacity. Determined by valence electrons.
- Radicals: Groups of atoms with a NET CHARGE. OH⁻, NO₃⁻, SO₄²⁻, NH₄⁺, CO₃²⁻, PO₄³⁻.
- Writing Formulae: Cross-multiply valencies. Simplify the ratio.
Chemical Equations — Balancing
Atoms CANNOT be created or destroyed. Number of atoms of EACH element must be the SAME on BOTH sides. Use COEFFICIENTS. Never change subscripts.
2. Chemical Changes and Reactions
Types of Reactions
| Type | Definition | Example |
|---|---|---|
| Combination | A + B → AB | 2H₂ + O₂ → 2H₂O |
| Decomposition | AB → A + B | CaCO₃ → CaO + CO₂ |
| Displacement | A + BC → AC + B | Zn + CuSO₄ → ZnSO₄ + Cu |
| Double Displacement | AB + CD → AD + CB | AgNO₃ + NaCl → AgCl↓ + NaNO₃ |
Energy Changes
- Exothermic: RELEASES heat. Combustion, respiration.
- Endothermic: ABSORBS heat. Photosynthesis, melting.
3. Water — The Universal Solvent
Properties
- Excellent SOLVENT — dissolves many substances. 'Water is called the UNIVERSAL SOLVENT.'
- High specific heat capacity — moderates CLIMATE.
- Anomalous expansion: Water EXPANDS on FREEZING (density < 1 g/cm³ → ice FLOATS). 'If ice sank, lakes would freeze from the BOTTOM UP — killing all aquatic life.'
Solutions
- Solute + Solvent = Solution.
- Saturated: No more solute can dissolve at that temperature.
- Solubility = Mass of solute that dissolves in 100 g of solvent at a given temperature.
Tests for Water
- Anhydrous COPPER SULPHATE: WHITE → BLUE in presence of water.
- Anhydrous COBALT CHLORIDE: BLUE → PINK.
4. Atomic Structure and Chemical Bonding
Bohr's Model (Review)
Electrons in shells. K(2), L(8), M(8), N(18). 'Atoms STABILIZE by achieving an OCTET (8) in the outermost shell.'
Chemical Bonding — The Two Types
| Ionic (Electrovalent) | Covalent | |
|---|---|---|
| How | TRANSFER of electron(s) from metal to non-metal | SHARING of electron(s) between non-metals |
| Force | Electrostatic attraction between OPPOSITELY charged ions | Shared electrons |
| Example | NaCl (Na⁺ Cl⁻). MgO. | H₂, Cl₂, H₂O, CH₄, CO₂ |
5. The Periodic Table
Mendeleev's Periodic Table
Arranged elements by INCREASING ATOMIC MASS. 'Left GAPS for undiscovered elements — and PREDICTED their properties. He was RIGHT. This was his GENIUS.'
Modern Periodic Law
'Properties of elements are a PERIODIC FUNCTION of their ATOMIC NUMBER.' NOT atomic mass. The correction was made by MOSELEY.
Structure of the Modern Periodic Table
- 7 PERIODS (horizontal rows). 18 GROUPS (vertical columns).
- PERIOD number = Number of SHELLS.
- GROUP number = Number of VALENCE ELECTRONS (for groups 1-2, 13-18).
Key Groups
| Group | Name | Characteristics |
|---|---|---|
| 1 | Alkali Metals (Li, Na, K...) | 1 valence electron. HIGHLY reactive. Form +1 ions. |
| 2 | Alkaline Earth Metals (Mg, Ca...) | 2 valence electrons. Reactive. Form +2 ions. |
| 17 | Halogens (F, Cl, Br...) | 7 valence electrons. VERY reactive. Form -1 ions. |
| 18 | Noble Gases (He, Ne, Ar...) | FULL outer shell. EXTREMELY UNREACTIVE. |
Periodic Trends
- Atomic size: DECREASES across period (more protons → stronger pull on electrons). INCREASES down group (more shells).
- Metallic character: DECREASES across period. INCREASES down group.
