1.6 Ionic BondingEdexcel IGCSE Chemistry: Revision notes
Section 1
How are ions formed?
An ion is a charged particle formed when an atom loses or gains electrons.
- Metal atoms lose electrons to form positively charged ions (cations), since metals have few outer-shell electrons and it takes less energy to lose them and achieve a full outer shell.
- Non-metal atoms gain electrons to form negatively charged ions (anions), since non-metals have nearly full outer shells and gaining electrons completes the shell.
In both cases, the resulting ion has the stable, full-outer-shell electron arrangement of the nearest noble gas.
Section 2
What are the charges of common ions?
| Ions | Charge |
|---|---|
| Group 1 metals (Li, Na, K) | 1+ |
| Group 2 metals (Mg, Ca) | 2+ |
| Group 3 metals (Al) | 3+ |
| Group 5 non-metals (N, P) | 3− |
| Group 6 non-metals (O, S) | 2− |
| Group 7 non-metals (Cl, Br, I) | 1− |
| Ag⁺, H⁺, NH₄⁺ | 1+ |
| Cu²⁺, Fe²⁺, Pb²⁺, Zn²⁺ | 2+ |
| Fe³⁺ | 3+ |
| OH⁻, NO₃⁻ | 1− |
| CO₃²⁻, SO₄²⁻ | 2− |
The charge of a main-group ion relates to its group number: metals in Groups 1–3 lose electrons equal to their group number; non-metals in Groups 5–7 gain electrons equal to (8 − group number).
Learn the transition metal ion charges (Cu²⁺, Fe²⁺/Fe³⁺, Zn²⁺, Pb²⁺, Ag⁺) individually — they can't be predicted from group number.
Section 3
How do you write formulae for ionic compounds?
To write a formula for an ionic compound, balance the total positive and negative charge so the overall charge is zero.
- Write the symbols/formulae of the ions with their charges.
- Find the lowest common multiple of the charges to balance them.
- Use subscripts (or brackets for polyatomic ions used more than once) to show the ratio.
For example, calcium (Ca²⁺) and chloride (Cl⁻): two chloride ions are needed to balance one calcium ion, giving CaCl₂. Calcium and carbonate (CO₃²⁻) balance 1:1, giving CaCO₃.
Aluminium (Al³⁺) and oxide (O²⁻): lowest common multiple of 3 and 2 is 6, so 2 Al³⁺ (total 6+) balance 3 O²⁻ (total 6−), giving Al2O3.
Section 4
How is ionic bonding represented and explained?
Dot-and-cross diagrams show ionic bond formation by electron transfer: electrons from the metal atom's outer shell are shown as dots (or crosses) transferred to complete the outer shell of the non-metal atom, with square brackets and charges drawn around each resulting ion.
Ionic bonding is the strong electrostatic force of attraction between oppositely charged ions, which acts in all directions and holds the ions together in a giant ionic lattice.
Section 5
Why do ionic compounds have high melting/boiling points and conduct only when molten or dissolved?
Ionic compounds form giant ionic lattices with very strong electrostatic attractions between ions acting in all directions. A large amount of energy is needed to overcome these forces, so ionic compounds have high melting and boiling points.
Ionic compounds do not conduct electricity when solid, because the ions are held in fixed positions in the lattice and cannot move to carry charge. When molten or dissolved in water (aqueous), the ions are free to move, so they can carry charge and the compound does conduct electricity.
Students often say ionic compounds 'don't conduct at all' — they must specify solid vs molten/aqueous, since conductivity depends on whether ions are free to move.
Must Know
- Ions form when atoms lose (cations, metals) or gain (anions, non-metals) electrons to reach a full outer shell
- Learn common ion charges: Group 1 (1+), Group 2 (2+), Group 3 (3+), Group 5 (3−), Group 6 (2−), Group 7 (1−), plus named transition metal/polyatomic ions
- Ionic formulae balance total positive and negative charge to zero
- Ionic bonding = strong electrostatic attraction between oppositely charged ions in a giant lattice
- Ionic compounds have high melting/boiling points due to strong lattice forces
- Ionic compounds conduct electricity only when molten or in aqueous solution, not when solid, because ions must be free to move
That's the notes covered.
Carry on to the next subtopic.