Ionic Bonding
Principles of Chemistry
The charges of a fixed set of common ions must be learnt by heart.
Positive ions (cations)
| Ion | Charge | Example |
|---|---|---|
| Group 1 metal | 1+ | Li⁺, Na⁺, K⁺ |
| Group 2 metal | 2+ | Mg²⁺, Ca²⁺ |
| Group 3 metal | 3+ | Al³⁺ |
| Silver | 1+ | Ag⁺ |
| Copper(II) | 2+ | Cu²⁺ |
| Iron(II) | 2+ | Fe²⁺ |
| Iron(III) | 3+ | Fe³⁺ |
| Lead(II) | 2+ | Pb²⁺ |
| Zinc | 2+ | Zn²⁺ |
| Hydrogen | 1+ | H⁺ |
| Ammonium | 1+ | NH₄⁺ |
Negative ions (anions)
| Ion | Charge | Example |
|---|---|---|
| Group 5 non-metal | 3− | N³⁻ |
| Group 6 non-metal | 2− | O²⁻, S²⁻ |
| Group 7 non-metal | 1− | F⁻, Cl⁻, Br⁻, I⁻ |
| Hydroxide | 1− | OH⁻ |
| Nitrate | 1− | NO₃⁻ |
| Carbonate | 2− | CO₃²⁻ |
| Sulfate | 2− | SO₄²⁻ |
- Polyatomic (NH₄⁺, OH⁻, NO₃⁻, CO₃²⁻, SO₄²⁻) are clusters of atoms that carry their charge as a single unit. Treat them as one piece in any formula you build
Common exam question
Giving the formula of each ion in a compound
Question: Give the formula of each ion in a named compound such as calcium nitrate, lead(II) nitrate or copper(II) sulfate (1–2 marks).
Asked in 4 of the 23 papers. Each ion needs its symbol with the full charge as a superscript, sign included: Ca²⁺ and NO₃⁻. A Roman numeral tells you the metal's charge, so lead(II) is Pb²⁺ and copper(II) is Cu²⁺. A polyatomic ion keeps its charge on the whole group, SO₄²⁻ rather than SO₄⁻, and a double salt such as KAl(SO₄)₂ splits into three ions, K⁺, Al³⁺ and SO₄²⁻, whose charges add up to zero. Writing a charge as a subscript, or a symbol in the wrong case, is capped at one mark in one scheme; writing the sign before the number, as in Al⁺³, is accepted.