What it is
An atom is neutral: as many protons (positive) as electrons (negative). If it gains or loses electrons it becomes an ion. The number of protons never changes in the process, only the electrons.
- A cation is a positive ion, with fewer electrons than protons. Na → Na⁺ + e⁻: sodium has 11 protons and is left with 10 electrons. Mg²⁺ has 12 protons and 10 electrons.
- An anion is a negative ion, with more electrons than protons. Cl⁻ has 17 protons and 18 electrons.
- Polyatomic ions are groups of atoms with a shared charge: NH₄⁺ (11 protons, 10 electrons) and SO₄²⁻ (48 protons, 50 electrons).
A salt such as NaCl consists of ions in a ratio that makes the total neutral: one Na⁺ per Cl⁻. In CaCl₂ there are two Cl⁻ for each Ca²⁺. Dissolve 0.10 mol/dm³ CaCl₂ in water and you get 0.10 mol/dm³ Ca²⁺ and 0.20 mol/dm³ Cl⁻, 0.30 mol/dm³ of ions altogether.
Not ions
- The sodium atom Na. 11 protons and 11 electrons: neutral.
- The water molecule H₂O. It is polar but has as many electrons as protons, so it has no net charge.
- Sugar molecules in a sucrose solution. Sugar dissolves without splitting into ions.
Water does contain a little H₃O⁺ and OH⁻ all the same, through the ion product of water.
Why it matters
- Solutions conduct electricity when they contain ions that can move. Salt water does, sugar water does not.
- Acids and bases. The hydrogen ion H⁺ is what an acid gives and a base takes, see acids and bases.
- Redox is electrons moving, which forms or uses up ions, see oxidation and reduction.
- Solubility. The solubility product works with the ion concentrations in a saturated solution, and the solubility rules say which ionic compounds dissolve.
- Concentration of an ion is given like any other substance, in mol/dm³, see concentration.
- Faraday’s law works out how much of the ions is converted at an electrode.
Common misconceptions
- “A cation is negative.” A cation is positive: it has lost electrons. An anion is negative.
- “Ions form when protons move.” The protons stay in the nucleus. It is electrons that move.
- “A salt is neutral, so there are no ions.” The salt is neutral as a whole, but made of positive and negative ions.
- “An ion and an isotope are the same thing.” An isotope differs in the number of neutrons; an ion in the number of electrons.
Common questions
How do I find an ion’s charge?
The charge is the number of protons minus the number of electrons. For Mg²⁺: 12 − 10 = +2.
Why does sodium form a +1 ion?
Sodium has one outer electron more than neon, which has a stable electron arrangement, and gives it up easily, so Na⁺ is common.
The periodic table shows where the elements sit.
What is the difference between an ion and an ionic substance?
An ion is a single charged particle. An ionic substance, such as NaCl, is a crystal lattice of positive and
negative ions with a net charge of zero.