1. Electrovalent (ionic) bonding
Atoms combine to reach a noble-gas arrangement: an octet (or a duplet for the first shell). An electrovalent bond is formed by transfer of electrons from a metal atom (low ionisation potential) to a non-metal atom (high electron affinity); the ions are held by electrostatic attraction. Electrovalency = number of electrons lost or gained.
| Compound | Transfer | Ions |
|---|---|---|
| NaCl | Na (2, 8, 1) → 1 e⁻ → Cl (2, 8, 7) | Na⁺ (2, 8), Cl⁻ (2, 8, 8) |
| MgCl₂ | Mg (2, 8, 2) → 1 e⁻ to each of 2 Cl | Mg²⁺, 2Cl⁻ |
| CaO | Ca (2, 8, 8, 2) → 2 e⁻ → O (2, 6) | Ca²⁺, O²⁻ (2, 8) |
In dot diagrams: crosses for the metal’s electrons, dots for the non-metal’s, square brackets and charges on the ions.
| Favours an ionic bond | Reason |
|---|---|
| Low ionisation potential of the metal | Loses electrons easily |
| High electron affinity of the non-metal | Gains electrons readily |
| Large electronegativity difference | Complete transfer rather than sharing |
Key terms
- Electrovalent bond
A bond formed by the transfer of electrons from one atom to another, the resulting ions being held by electrostatic attraction.
- Electrovalency
The number of electrons an atom loses or gains in forming an ionic bond.
2. Properties of electrovalent compounds
- Hard crystalline solids: ions in a lattice held by strong forces.
- High melting and boiling points (NaCl melts at 801 °C).
- Soluble in water, insoluble in organic solvents (kerosene, petrol).
- Conduct only when molten or in aqueous solution: the compound dissociates into free ions (NaCl → Na⁺ + Cl⁻), which carry current and are discharged at the electrodes — electrolysis. Solid: ions fixed, no conduction.
| State | Conducts? |
|---|---|
| Solid NaCl | No |
| Molten NaCl | Yes |
| NaCl solution | Yes |
Formulas
- Dissociation of molten magnesium chloride
Key terms
- Dissociation
The separation of an electrovalent compound into its free ions when melted or dissolved.
3. Covalent bonding: non-polar molecules
A covalent bond is a shared pair of electrons, one electron from each atom. Single bond = 1 pair, double = 2 pairs, triple = 3 pairs. Lone pairs are unshared valence pairs.
| Molecule | Shared pairs | Lone pairs |
|---|---|---|
| H₂ | 1 (H–H) | 0 |
| Cl₂ | 1 (Cl–Cl) | 3 on each Cl |
| O₂ | 2 (O=O) | 2 on each O |
| N₂ | 3 (N≡N) | 1 on each N |
| CH₄ | 4 C–H | 0 |
| CCl₄ | 4 C–Cl | 3 on each Cl |
Identical atoms share equally (non-polar). CH₄ and CCl₄ are symmetrical (tetrahedral), so they are non-polar molecules.
| Bond | Pairs | Example |
|---|---|---|
| Single | 1 | Cl₂ |
| Double | 2 | O₂ |
| Triple | 3 | N₂ |
Key terms
- Covalent bond
A bond formed by the sharing of a pair of electrons between two atoms, each contributing one electron.
- Lone pair
A pair of valence electrons on an atom that is not used in bonding.
4. Polar covalent molecules
When different atoms share a pair, the more electronegative atom attracts it more strongly and carries δ−; the other carries δ+: a polar covalent bond.
- HCl: H(δ+)–Cl(δ−); one shared pair, three lone pairs on Cl.
- H₂O: two O–H bonds, two lone pairs on O; bent; polar.
- NH₃: three N–H bonds, one lone pair on N; pyramidal; polar.
| Molecule | Bond pairs | Lone pairs on central atom |
|---|---|---|
| HCl | 1 | 3 (on Cl) |
| H₂O | 2 | 2 |
| NH₃ | 3 | 1 |
Key terms
- Polar covalent bond
A covalent bond in which the shared pair lies closer to the more electronegative atom, giving partial charges.
5. Electrovalent and covalent compounds compared
| Property | Electrovalent | Covalent |
|---|---|---|
| Particles | Ions | Molecules |
| State | Hard crystalline solids | Gases, liquids, soft solids |
| Melting / boiling point | High | Low |
| Solubility | Water | Organic solvents |
| Conductivity | Molten or aqueous: yes; solid: no | Non-conductors |
Some polar covalent compounds ionise in water and conduct: .
| Substance | Bonding |
|---|---|
| KCl | Electrovalent |
| CCl₄ | Covalent (non-polar) |
| HCl | Covalent (polar) |
Formulas
- HCl ionises in water
Key terms
- Ionisation (of a polar compound)
Formation of ions when a polar covalent compound such as HCl reacts with water.
6. Coordinate bonding
A coordinate (dative) bond is a shared pair in which both electrons come from one atom (the donor, which has a lone pair); the acceptor (here H⁺) has none.
- (hydronium ion: 2 covalent + 1 coordinate bond)
- (ammonium ion: 3 covalent + 1 coordinate bond)
- Water self-ionises: — the lone pair on one water molecule takes H⁺ from another.
| Ion | Donor | Acceptor |
|---|---|---|
| H₃O⁺ | O of H₂O | H⁺ |
| NH₄⁺ | N of NH₃ | H⁺ |
Formulas
- Ammonium ion
Key terms
- Coordinate bond
A covalent bond in which both electrons of the shared pair are donated by one atom.
Exam tips
Common mistakes
- Drawing inner-shell electrons in dot diagrams, or forgetting brackets and charges on ions.
- Saying solid sodium chloride conducts.
- Drawing O₂ with a single bond or N₂ without lone pairs.
- Calling CCl₄ polar.
How marks are usually awarded
- Dot diagram (2 marks): correct electrons on each atom, and correct ions or shared pairs.
- Properties (1 mark each): property with the reason (ions fixed or free; weak forces between molecules).
- Coordinate bond (2 marks): lone pair donated, product ion with charge.