What are cation and anion?

1. Save ink with cation and anion

Cation and anion are cheem words that look like typos: vacation with its va on vacation and onion that is misspelled. But they exist for a reason. They provide a quicker way of saying positively charged ion and negatively charged ion.

2. How to find the charge of a cation?

Hydrogen	Group I metals	Group II metals	Group III metals
H+	Li+ Na+ K+	Be2+ Mg2+ Ca2+	Al3+

A hydrogen atom or a metallic atom can lose its valence electrons to form a cation. As it loses electrons, there will be more protons than the remaining electrons, hence increasing the magnitude of the positive charge.

Therefore, the charges of cations will depend on how many valence electrons they had to lose. For metallic cations in Groups I, II, and III, their charges correspond with the group number.

3. Transition metals (and lead) cations are rule breakers

Charge = 1+	Charge = 2+	Charge = 3+
Copper(I), Cu+ Silver, Ag+	Copper(II), Cu2+ Iron(II), Fe2+ Zinc, Zn2+	Iron(III), Fe3+

Unfortunately, transition metals don’t have such a neat rule. Some transition metals can lose different number of electrons at different times, forming cations of different charges.

To avoid confusion, we sometimes indicate the charge of transition metal cation with the roman numeral behind its name. For example, iron(II) cation has a charge of 2+.

Confusing the matter further, lead in Group IV can also form ions of different charges: lead(II) and lead(IV) ions. However, only lead(II) ions are featured in the O Level as they are way more common.

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4. A cation can also be made up of a few atoms: ammonium cation

Cations can also be polyatomic, if they are made up of multiple atoms covalently bonded. The only polyatomic cation that you need to memorise is the ammonium cation. Its chemical formula is NH₄⁺, which shows that each ion is made up of 1 nitrogen atom and 4 hydrogen atoms. These atoms are covalently bonded, and they share a single positive charge.

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5. How to find the charge of an anion?

Group V metals	Group VI metals	Group VII metals
Nitride, N3-	Oxide, O2- Sulfide, S2-	Fluoride, F– Chloride, Cl– Bromide, Br– Iodide, I–

Conversely, a non-metallic atom can gain electrons to form an anion. The charge of the anion is equal to the number of electrons gained.

Generally, non-metallic atoms tend to gain electrons until there are 8 valence electrons. Therefore, the magnitude of the negative charge is the difference between 8 and the group number.

6. There are also polyatomic anions

Charge = 1-	Charge = 2-
Hydroxide, OH– Nitrate, NO3– Hydrogencarbonate, HCO3–	Carbonate, CO32- Sulfite, SO32- Sulfate, SO42-

Some anions are made up of multiple atoms that are covalently bonded together. They are called polyatomic anions.

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7. The cations and anions tested in the O Level

It’s time to test yourself! Do you know the charge of each cation and anion? Challenge yourself further by writing the chemical formula of each ion.