Ionic Bonding & Ionic Compounds

How ions form

Atoms are most stable when they have a full outer shell of electrons (the same arrangement as a noble gas). Most atoms do not start out this way, so when they react they lose or gain electrons to reach a full shell. Charged particles formed in this way are called ions.

Metals are on the left of the Periodic Table and have few outer electrons. It is easier for them to lose these electrons. Losing negative electrons leaves a positive ion.
Non-metals are on the right and have nearly full outer shells. It is easier for them to gain electrons. Gaining negative electrons makes a negative ion.

Key terms

Ion – a charged particle made when an atom (or group of atoms) loses or gains electrons.

Cation – a positive ion, formed by losing electrons (e.g. $Na^+$).

Anion – a negative ion, formed by gaining electrons (e.g. $Cl^-$).

A sodium atom has the electronic configuration 2,8,1. By losing its single outer electron it becomes $Na^+$ with the configuration 2,8 — the same as neon. A chlorine atom (2,8,7) gains one electron to become $Cl^-$ (2,8,8) — the same as argon.

The charges of common ions

For ions in the main groups, the charge follows the group number. Metals in groups 1, 2 and 3 form positive ions; non-metals in groups 5, 6 and 7 form negative ions. Group 0 (the noble gases) already have full shells, so they do not normally form ions.

Some common compound ions carry their charge across a group of atoms and should simply be learned: $OH^-$ (hydroxide), $NO_3^-$ (nitrate), $CO_3^{2-}$ (carbonate), $SO_4^{2-}$ (sulfate) and $NH_4^+$ (ammonium).

Ionic bonding

When a metal reacts with a non-metal, electrons are transferred from the metal to the non-metal. This produces oppositely charged ions which strongly attract one another.

Key terms

Ionic bond – the strong electrostatic attraction between oppositely charged ions.

The diagram below shows electron transfer in sodium chloride. We draw the outer shell only, using dots for one atom's electrons and crosses for the other so we can track where each electron goes.

For magnesium oxide (MgO), magnesium (group 2) loses two electrons and oxygen (group 6) gains two, giving $Mg^{2+}$ and $O^{2-}$. For magnesium chloride (MgCl₂), magnesium loses two electrons but each chlorine atom can only gain one, so two chlorine atoms are needed — written $[Mg]^{2+}\,2[Cl]^-$.

Exam tip

In a dot-and-cross diagram for an ionic compound, draw each ion in square brackets with the charge written outside the top-right corner, e.g. $[Mg]^{2+}$. The metal ion is usually drawn with an empty outer shell (it gave its electrons away).

The giant ionic lattice

Ionic compounds do not exist as single pairs of ions. Billions of oppositely charged ions pack together in a regular, repeating 3-D arrangement called a giant ionic lattice. Each ion is surrounded by ions of the opposite charge, so the attractions act in all directions.

Properties of ionic compounds

The strong attractions in the lattice explain how ionic compounds behave.

Watch out

A solid ionic compound does not conduct electricity. The ions are present but locked in the lattice. Only say a compound conducts when it is molten or dissolved in water — and always link this to ions being free to move.

Working out ionic formulae

The overall charge on an ionic compound is zero, so the positive and negative charges must balance. A quick method is to swap the charges and use them as the numbers of each ion (the "cross-over" rule), then cancel down.

Worked example

Find the formula of aluminium oxide.

Step 1 – write the ions: aluminium is in group 3, so $Al^{3+}$; oxygen is in group 6, so $O^{2-}$.

Step 2 – balance the charges. Two $Al^{3+}$ give $+6$; three $O^{2-}$ give $-6$. The charges now cancel.

A few more worked formulae:

Sodium chloride: $Na^+$ and $Cl^-$ → one of each → $NaCl$.
Calcium chloride: $Ca^{2+}$ and $Cl^-$ → need two $Cl^-$ → $CaCl_2$.

Exam tip

When a compound ion like $OH^-$ or $NO_3^-$ is needed more than once, put it in brackets before the subscript: $Ca(OH)_2$, $Mg(NO_3)_2$. Writing $CaOH_2$ would be marked wrong.