Ionic bonding

What ionic bonding is

• Ionic bonding is the strong electrostatic attraction between oppositely charged ions.
• It forms between a metal and a non-metal.
• Electrons are transferred from the metal to the non-metal — not shared.

How the ions form

• Atoms are stable when their outer shell is full (usually 8 electrons — an octet).
• A metal has few outer electrons, so the quickest route to a full shell is to lose them.
  • Losing negative electrons leaves the atom positively charged — a cation.
• A non-metal has nearly a full outer shell, so its quickest route is to gain electrons.
  • Gaining negative electrons makes the atom negatively charged — an anion.

Worked through for sodium chloride:

• Sodium is 2,8,1 — one outer electron. It loses it → $Na^+$, leaving a full 2,8 shell.
• Chlorine is 2,8,7 — one short of a full shell. It gains that electron → $Cl^-$, giving 2,8,8.
• The $Na^+$ and $Cl^-$ now attract each other. That attraction is the ionic bond.

Working out the charge on an ion

• Group 1 metals lose 1 electron → 1+ ($Na^+$, $K^+$)
• Group 2 metals lose 2 → 2+ ($Mg^{2+}$, $Ca^{2+}$)
• Group 13 metals lose 3 → 3+ ($Al^{3+}$)
• Group 16 non-metals gain 2 → 2− ($O^{2-}$, $S^{2-}$)
• Group 17 non-metals gain 1 → 1− ($Cl^-$, $Br^-$)

From charges to a formula

• The compound must be electrically neutral overall, so the total positive charge must equal the total negative charge.
• Find the charge on each ion.
• Balance them with the smallest whole numbers.
• Write the metal first, then the non-metal, with subscripts.

Worked through:

• Magnesium oxide — $Mg^{2+}$ and $O^{2-}$. The 2+ and 2− already cancel, so MgO.
• Sodium oxide — $Na^+$ and $O^{2-}$. You need two $Na^+$ to cancel one $O^{2-}$, so $Na_2O$.
• Aluminium chloride — $Al^{3+}$ and $Cl^-$. You need three $Cl^-$, so $AlCl_3$.

Why it forms a lattice, not a pair

• The attraction between an ion and an opposite charge acts in every direction at once.
• So an ion does not pair off with one partner — it attracts all its oppositely charged neighbours.
• The ions keep stacking into a giant ionic lattice, a regular repeating three-dimensional arrangement.

• There are no molecules in an ionic compound. "NaCl" is the ratio of ions, not a particle you could pick out.
• The lattice is held together by very many strong attractions, which is why ionic compounds have high melting points.