Ionic bonding: the formation of ions by electron transfer, dot-and-cross diagrams, the giant ionic lattice, and how the structure explains the properties of ionic compounds.

What this dot point is asking

Edexcel wants you to explain how ionic bonds form when a metal transfers electrons to a non-metal, work out the charges and formulae of the ions, draw dot-and-cross diagrams, describe the giant ionic lattice, and use that structure to explain the characteristic properties of ionic compounds (high melting point, brittleness, and conduction only when molten or in solution). It is one of the three bonding types you must compare.

How ions form by electron transfer

Atoms are most stable with a full outer shell (the electronic configuration of a noble gas). Metals have few outer electrons and lose them; non-metals have nearly full outer shells and gain electrons.

• A metal atom loses electrons to form a positive ion (cation). For example, sodium $2,8,1$ loses one electron to become $Na^+$ with configuration $2,8$.
• A non-metal atom gains electrons to form a negative ion (anion). For example, chlorine $2,8,7$ gains one electron to become $Cl^-$ with configuration $2,8,8$.

The charge on the ion equals the number of electrons gained or lost. Group 1 ions are $+1$, Group 2 are $+2$, Group 3 are $+3$; Group 7 ions are $-1$, Group 6 are $-2$.

Dot-and-cross diagrams

A dot-and-cross diagram shows where the electrons come from. The metal's electrons are drawn as dots and the non-metal's as crosses (or vice versa). After transfer, each ion is drawn in square brackets with its charge outside, for example $[Na]^+$ and $[Cl]^-$ each with a full outer shell. Only the outer shells need to be shown for the transfer to be clear.

The giant ionic lattice

Ionic compounds do not exist as separate molecules. Instead the ions pack together in a regular, repeating three-dimensional arrangement called a giant ionic lattice, with each ion surrounded by ions of the opposite charge.

Explaining the properties

The lattice structure explains every typical property:

• High melting and boiling points. The electrostatic forces between ions are strong and numerous, so a large amount of energy is needed to separate the ions. The greater the ionic charges, the stronger the attraction and the higher the melting point (magnesium oxide, $2+$ and $2-$, melts far higher than sodium chloride, $1+$ and $1-$).
• Do not conduct when solid. The ions are locked in fixed positions and cannot move, so there are no mobile charge carriers.
• Conduct when molten or dissolved. Melting or dissolving frees the ions to move, so they can carry charge.
• Brittle. A blow can shift one layer so that ions of the same charge line up; they repel, and the crystal splits.

Try this

Q1. State the charge on an ion formed by a Group 2 metal and on an ion formed by a Group 7 non-metal. [2 marks]

• Cue. Group 2 ion $2+$; Group 7 ion $1-$.

Q2. Potassium ($2,8,8,1$) reacts with oxygen. Describe the electron transfer and give the formula of potassium oxide. [3 marks]

• Cue. Each potassium loses one electron to form $K^+$; each oxygen gains two to form $O^{2-}$; two $K^+$ balance one $O^{2-}$, giving $K_2O$.

Q3. Explain why sodium chloride conducts electricity when dissolved in water but not when solid. [2 marks]

• Cue. When dissolved the ions are free to move and carry charge; when solid the ions are held in fixed positions and cannot move.