Describe ionic bonding as electron transfer, draw dot-and-cross diagrams for simple ionic compounds, and relate the giant ionic lattice to the properties of ionic compounds.

What this topic is asking

WJEC topic 2.1 wants you to explain ionic bonding as the transfer of electrons from a metal to a non-metal, to draw dot-and-cross diagrams for simple ionic compounds, and to relate the giant ionic lattice to the properties of ionic compounds (high melting points and conduction only when molten or dissolved).

How an ionic bond forms

A metal atom has only a few outer electrons, which it loses to form a positive ion (cation). A non-metal atom has a nearly full outer shell, which it completes by gaining electrons to form a negative ion (anion). Both ions end up with a full, stable outer shell like a noble gas.

For example, magnesium $2,8,2$ loses two electrons to become $\text{Mg}^{2+}$ ($2,8$), and oxygen $2,6$ gains two electrons to become $\text{O}^{2-}$ ($2,8$), forming magnesium oxide, $\text{MgO}$.

Dot-and-cross diagrams

A dot-and-cross diagram shows where the electrons come from. The electrons of one atom are drawn as dots and the other as crosses, so you can see the transfer. For sodium chloride:

• Draw sodium losing its one outer electron, leaving $\text{Na}^+$ with a full shell.
• Draw chlorine gaining that electron, giving $\text{Cl}^-$ with a full outer shell of 8.
• Show the charges on each ion in square brackets, for example $[\text{Na}]^+$ and $[\text{Cl}]^-$.

The giant ionic lattice

Ionic compounds do not exist as single molecules. Instead, billions of ions arrange themselves in a regular, repeating 3D pattern called a giant ionic lattice, in which each positive ion is surrounded by negative ions and vice versa. The strong electrostatic forces act in all directions throughout the lattice.

Properties of ionic compounds

The giant lattice explains the typical properties:

Ionic compounds are also usually soluble in water and form brittle crystals (a blow can shift layers so like charges align and repel, shattering the crystal).

It helps to remember that an ionic formula such as $\text{NaCl}$ does not mean a single molecule of one sodium and one chlorine. It is the simplest whole-number ratio of ions in the giant lattice (one $\text{Na}^+$ for every $\text{Cl}^-$). The whole crystal is held together by the same kind of strong electrostatic attraction repeated billions of times, which is why even a tiny grain of salt is extremely strongly bonded and needs a high temperature to melt.