Language of Chemistry
1. Valency
VALENCY is the COMBINING CAPACITY of an element. It is the number of electrons an atom GAINS, LOSES, or SHARES to become stable (complete octet/duplet).
Determining Valency
| Type | Valency | Examples |
|---|---|---|
| Elements with 1 valence electron | 1 | H, Na, K, Li |
| Elements with 2 valence electrons | 2 | Mg, Ca, Zn |
| Elements with 3 valence electrons | 3 | Al |
| Elements with 4 valence electrons | 4 | C, Si |
| Elements with 5 valence electrons | 3 | N, P |
| Elements with 6 valence electrons | 2 | O, S |
| Elements with 7 valence electrons | 1 | F, Cl, Br |
| Elements with 8 valence electrons (Noble gases) | 0 | He, Ne, Ar |
Variable Valency
Some elements show MORE than one valency.
| Element | Valencies | Example Compounds |
|---|---|---|
| Iron (Fe) | 2, 3 | FeCl₂, FeCl₃ |
| Copper (Cu) | 1, 2 | Cu₂O, CuO |
| Mercury (Hg) | 1, 2 | Hg₂Cl₂, HgCl₂ |
| Sulphur (S) | 2, 4, 6 | H₂S, SO₂, SO₃ |
Roman Numeral Notation
Variable valency is shown with Roman numerals:
- Fe²⁺ = Iron(II), Fe³⁺ = Iron(III).
- Cu⁺ = Copper(I), Cu²⁺ = Copper(II).
2. Radicals (Polyatomic Ions)
A RADICAL is a group of atoms that behaves as a SINGLE unit with a NET CHARGE.
Common Positive Radicals (Cations)
| Radical | Symbol | Valency |
|---|---|---|
| Ammonium | NH₄⁺ | 1 |
| Hydrogen | H⁺ | 1 |
Common Negative Radicals (Anions)
| Radical | Symbol | Valency |
|---|---|---|
| Hydroxide | OH⁻ | 1 |
| Nitrate | NO₃⁻ | 1 |
| Bicarbonate | HCO₃⁻ | 1 |
| Carbonate | CO₃²⁻ | 2 |
| Sulphate | SO₄²⁻ | 2 |
| Sulphite | SO₃²⁻ | 2 |
| Phosphate | PO₄³⁻ | 3 |
3. Writing Chemical Formulae
Criss-Cross Method
- Write the symbols of the elements/radicals.
- Write their valencies ABOVE the symbols.
- CROSS the valencies (valency of first becomes subscript of second, and vice versa).
- Simplify if needed (divide by common factor).
Examples
Sodium chloride: Na¹⁺ Cl¹⁻ → NaCl Magnesium chloride: Mg²⁺ Cl¹⁻ → MgCl₂ Calcium oxide: Ca²⁺ O²⁻ → CaO (simplified from Ca₂O₂) Aluminium oxide: Al³⁺ O²⁻ → Al₂O₃ Sodium sulphate: Na¹⁺ SO₄²⁻ → Na₂SO₄ Calcium hydroxide: Ca²⁺ OH¹⁻ → Ca(OH)₂ Ammonium sulphate: NH₄¹⁺ SO₄²⁻ → (NH₄)₂SO₄
Worked Example (ICSE 2024, 2 marks)
'Write the formula of magnesium hydroxide.'
Solution: Mg²⁺ and OH¹⁻ → Mg(OH)₂.
4. Writing Chemical Equations
A CHEMICAL EQUATION represents a chemical reaction using SYMBOLS and FORMULAE.
Skeleton Equation
Reactants → Products Example: H₂ + O₂ → H₂O (not balanced).
Balanced Chemical Equation
The NUMBER of atoms of each element MUST be the SAME on both sides.
Rules for Balancing:
- Write the skeleton equation with correct formulae.
- Count atoms of each element on both sides.
- Add COEFFICIENTS (numbers before formulae) to balance.
- NEVER change the SUBSCRIPTS of a formula.
Examples
Hydrogen + Oxygen → Water: 2H₂ + O₂ → 2H₂O. Magnesium + Oxygen → Magnesium oxide: 2Mg + O₂ → 2MgO. Sodium + Water → Sodium hydroxide + Hydrogen: 2Na + 2H₂O → 2NaOH + H₂.
Information Conveyed by a Chemical Equation
- Reactants and products.
- Number of molecules (coefficients).
- States of substances: (s) solid, (l) liquid, (g) gas, (aq) aqueous.
- Direction of reaction (→ for irreversible, ⇌ for reversible).
5. Writing Word Equations
A WORD EQUATION uses the NAMES of substances instead of symbols.
Example: 'Zinc reacts with sulphuric acid to produce zinc sulphate and hydrogen.' Word: Zinc + Sulphuric acid → Zinc sulphate + Hydrogen. Formula: Zn + H₂SO₄ → ZnSO₄ + H₂.
6. ICSE Exam Focus
| Topic | Marks | Frequency |
|---|---|---|
| Valency determination | 2 marks | Very High |
| Writing chemical formulae (criss-cross) | 3 marks | Very High |
| Balancing simple equations | 3 marks | Very High |
| Radicals (names and symbols) | 2 marks | High |
| Word equations | 2 marks | Medium |
Common Mistakes
- Changing subscripts while balancing (only coefficients can be changed).
- Forgetting brackets for polyatomic ions: Ca(OH)₂ NOT CaOH₂.
- Writing hydrogen as H instead of H₂ (hydrogen exists as diatomic molecule).
- Confusing sulphate (SO₄²⁻) and sulphite (SO₃²⁻).
Self-Test (5 Questions)
Q1. What is the valency of aluminium? (1 mark)
- A) 1
- B) 2
- C) 3
- D) 4
Q2. Write the formula of: (i) Calcium carbonate (ii) Sodium hydroxide. (2 marks)
Q3. Write the formula of aluminium sulphate using the criss-cross method. (2 marks)
Q4. Balance: Fe + H₂O → Fe₃O₄ + H₂. (2 marks)
Q5. 'What is a radical? Give an example of a divalent negative radical.' (2 marks)
Answers
A1. C) 3. A2. (i) CaCO₃ (ii) NaOH. A3. Al³⁺ and SO₄²⁻ → Al₂(SO₄)₃. A4. 3Fe + 4H₂O → Fe₃O₄ + 4H₂. A5. A radical is a group of atoms that behaves as a single unit with net charge. Divalent negative radical: CO₃²⁻ (carbonate) or SO₄²⁻ (sulphate).
