Ions
An ion is an atom that has gained or lost electrons, so it has a charge.
- Losing electrons leaves more protons than electrons: a positive ion, or cation.
- Gaining electrons gives more electrons than protons: a negative ion, or anion.
charge = protons - electrons
Atoms tend to gain or lose electrons to reach a full outer level. Metals have few valence electrons and lose them. Nonmetals have many and gain a few more.
| Group | Valence electrons | Typical ion |
|---|---|---|
| 1 | 1 | 1+ |
| 2 | 2 | 2+ |
| 13 | 3 | 3+ |
| 15 | 5 | 3- |
| 16 | 6 | 2- |
| 17 | 7 | 1- |
Worked example: magnesium
Magnesium is in group 2, with 2 valence electrons. Losing both leaves 12 protons and 10 electrons.
12 - 10 = +2, so the ion is Mg2+
Common mistake
Thinking a positive ion gained something. A positive charge means electrons were lost. Protons never change when an ion forms.