Language of Chemistry
1. Valency
Valency is the COMBINING capacity of an element. It is the number of hydrogen atoms that can COMBINE with or DISPLACE one atom of the element.
Variable Valency
Some elements show MORE than one valency.
| Element | Valencies | Examples |
|---|---|---|
| Iron (Fe) | 2 (Ferrous), 3 (Ferric) | FeCl₂, FeCl₃ |
| Copper (Cu) | 1 (Cuprous), 2 (Cupric) | Cu₂O, CuO |
| Mercury (Hg) | 1 (Mercurous), 2 (Mercuric) | Hg₂Cl₂, HgCl₂ |
| Tin (Sn) | 2 (Stannous), 4 (Stannic) | SnCl₂, SnCl₄ |
| Lead (Pb) | 2 (Plumbous), 4 (Plumbic) | PbO, PbO₂ |
2. Radicals
A radical is a group of atoms that behaves as a SINGLE unit with a specific valency (charge).
Positive Radicals (Cations)
| Radical | Symbol | Valency |
|---|---|---|
| Hydrogen | H⁺ | 1 |
| Sodium | Na⁺ | 1 |
| Potassium | K⁺ | 1 |
| Silver | Ag⁺ | 1 |
| Ammonium | NH₄⁺ | 1 |
| Calcium | Ca²⁺ | 2 |
| Magnesium | Mg²⁺ | 2 |
| Zinc | Zn²⁺ | 2 |
| Aluminium | Al³⁺ | 3 |
Negative Radicals (Anions)
| Radical | Symbol | Valency |
|---|---|---|
| Chloride | Cl⁻ | 1 |
| Bromide | Br⁻ | 1 |
| Iodide | I⁻ | 1 |
| Hydroxide | OH⁻ | 1 |
| Nitrate | NO₃⁻ | 1 |
| Hydrogen carbonate | HCO₃⁻ | 1 |
| Oxide | O²⁻ | 2 |
| Sulphide | S²⁻ | 2 |
| Sulphate | SO₄²⁻ | 2 |
| Carbonate | CO₃²⁻ | 2 |
| Nitride | N³⁻ | 3 |
| Phosphate | PO₄³⁻ | 3 |
3. Writing Chemical Formulae
Criss-Cross Method:
- Write the symbols of the cation (positive) and anion (negative).
- Write their valencies ABOVE the symbols.
- CRISS-CROSS the valencies.
- SIMPLIFY the ratio if possible.
- Write the formula — cation FIRST.
Worked Example: Write the formula of Magnesium Chloride.
Mg (valency 2) and Cl (valency 1) Mg²⁺ Cl¹⁻ Criss-cross: MgCl₂
Worked Example: Write the formula of Aluminium Oxide.
Al (valency 3) and O (valency 2) Al³⁺ O²⁻ Criss-cross: Al₂O₃
Worked Example: Write the formula of Calcium Hydroxide.
Ca (valency 2) and OH (valency 1) Ca²⁺ OH¹⁻ Criss-cross: Ca(OH)₂ (brackets needed for POLYATOMIC radicals)
4. Balancing Chemical Equations
A chemical equation must be BALANCED — the SAME number of atoms of each element on BOTH sides.
Steps:
- Write the SKELETON equation (reactants → products).
- Count atoms on BOTH sides.
- Add COEFFICIENTS (numbers BEFORE formulas) to BALANCE the atoms.
- BALANCE ONE element at a time.
- Check that ALL elements are balanced.
Worked Example: Balance H₂ + O₂ → H₂O
H₂ + O₂ → H₂O H: 2 on LHS, 2 on RHS (correct) O: 2 on LHS, 1 on RHS (wrong) Add coefficient 2 before H₂O: H₂ + O₂ → 2H₂O H: 2 on LHS, 4 on RHS (wrong) Add coefficient 2 before H₂: 2H₂ + O₂ → 2H₂O H: 4 on LHS, 4 on RHS (correct) O: 2 on LHS, 2 on RHS (correct) BALANCED.
Worked Example: Balance Mg + O₂ → MgO
Mg + O₂ → MgO O: 2 on LHS, 1 on RHS → 2MgO on RHS Mg + O₂ → 2MgO Mg: 1 on LHS, 2 on RHS → 2Mg on LHS 2Mg + O₂ → 2MgO (correct)
5. Common Ions and Their Colours
| Ion | Colour |
|---|---|
| Copper(II) (Cu²⁺) | BLUE |
| Iron(II) (Fe²⁺) | PALE GREEN |
| Iron(III) (Fe³⁺) | YELLOW/BROWN |
| Permanganate (MnO₄⁻) | PURPLE |
| Dichromate (Cr₂O₇²⁻) | ORANGE |
| Chromate (CrO₄²⁻) | YELLOW |
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| 'Writing Fe₂(SO₄)₃ as Fe₂SO₄₃' | Bracket the polyatomic radical: Fe₂(SO₄)₃. The subscript 3 applies to the ENTIRE sulphate group |
| 'NaCl₂ is correct' | Na has valency 1, Cl has valency 1. Formula is NaCl. NaCl₂ is WRONG |
| 'Changing the subscript when balancing' | NEVER change subscripts. Only add COEFFICIENTS before formulas |
| 'Forgetting to simplify the criss-cross ratio' | Mg²⁺ O²⁻ → Mg₂O₂ → Simplify to MgO (divide by 2) |
ICSE Exam Focus (6–8 marks)
- 2-mark questions: Define valency, give examples of variable valency
- 3-mark questions: Write chemical formulae of given compounds
- 4-mark questions: Balance chemical equations
- 6-mark questions: Writing formulae and balancing combined
Self-Test
Q1. Write the formula of Sodium Carbonate. A1. Na⁺ (valency 1) CO₃²⁻ (valency 2). Criss-cross: Na₂CO₃.
Q2. Write the formula of Ammonium Sulphate. A2. NH₄⁺ (valency 1) SO₄²⁻ (valency 2). Criss-cross: (NH₄)₂SO₄.
Q3. Balance: Fe + H₂O → Fe₃O₄ + H₂ A3. 3Fe + 4H₂O → Fe₃O₄ + 4H₂. Check: Fe: 3=3, H: 8=8, O: 4=4 (correct).
Q4. What is variable valency? Give an example. A4. Some elements show MORE THAN ONE valency. Example: Iron shows valency 2 (FeCl₂ — Ferrous) and valency 3 (FeCl₃ — Ferric).
Q5. Why are brackets used in writing Ca(OH)₂? A5. Brackets are used when a POLYATOMIC radical (OH) appears more than once. Ca(OH)₂ means 2 OH groups. Without brackets, CaOH₂ would be incorrect.
Q6. Balance: Al + HCl → AlCl₃ + H₂ A6. 2Al + 6HCl → 2AlCl₃ + 3H₂. Check: Al: 2=2, H: 6=6, Cl: 6=6 (correct).
