Describe ionic bonding as the transfer of electrons to form ions with noble-gas configurations, draw dot-and-cross diagrams, and relate the giant ionic lattice to the properties of ionic compounds

What this dot point is asking

SEAB wants you to describe ionic bonding as the transfer of electrons from a metal to a non-metal so that both reach a stable noble-gas configuration, draw the dot-and-cross diagrams that show this, and relate the giant ionic lattice structure to the characteristic properties of ionic compounds (high melting point, conduction only when molten or dissolved). This is the first of the three bonding types and the model for electron transfer.

The answer

Ionic bonding as electron transfer

Atoms are most stable with a full outer shell (a noble-gas configuration). A metal atom has a few outer electrons it can lose; a non-metal atom needs a few electrons to fill its outer shell. In ionic bonding, the metal transfers its outer electrons to the non-metal:

• The metal atom loses electrons to become a positive ion (cation).
• The non-metal atom gains those electrons to become a negative ion (anion).

Both ions now have full outer shells. The oppositely charged ions attract each other by strong electrostatic forces, and this attraction is the ionic bond.

Working out the charge on an ion

The charge equals the number of electrons gained or lost to reach a full shell:

• Group I metals lose $1$ electron: charge $+1$ (such as $\text{Na}^+$).
• Group II metals lose $2$ electrons: charge $+2$ (such as $\text{Mg}^{2+}$).
• Group VI non-metals gain $2$ electrons: charge $-2$ (such as $\text{O}^{2-}$).
• Group VII non-metals gain $1$ electron: charge $-1$ (such as $\text{Cl}^-$).

The formula of the compound is found by balancing the total positive and negative charge, so magnesium chloride is $\text{MgCl}_2$ (one $\text{Mg}^{2+}$ needs two $\text{Cl}^-$).

Dot-and-cross diagrams

A dot-and-cross diagram shows the outer-shell electrons of the metal as crosses and of the non-metal as dots (or vice versa). For sodium chloride, the single sodium outer electron is transferred into the chlorine outer shell; the diagram shows $\text{Na}^+$ with an empty outer shell (or the shell below now outermost) and $\text{Cl}^-$ with eight outer electrons, each ion drawn in square brackets with its charge.

The giant ionic lattice and properties

An ionic compound is not made of molecules. It is a giant ionic lattice: a regular three-dimensional arrangement of huge numbers of alternating positive and negative ions, held together by strong electrostatic forces in all directions. This structure explains the properties:

• High melting and boiling points: much energy is needed to overcome the strong forces throughout the lattice. Ions with higher charges (such as $\text{Mg}^{2+}$ and $\text{O}^{2-}$) attract more strongly and melt at even higher temperatures.
• Conducts when molten or dissolved, but not when solid: conduction needs mobile charged particles. The ions are locked in place in the solid, but are free to move when the lattice is melted or the compound dissolves in water.
• Often soluble in water and brittle.

Examples in context

Example 1. Why sodium chloride dissolves and conducts. When table salt dissolves in water, the giant lattice breaks up and the $\text{Na}^+$ and $\text{Cl}^-$ ions become free to move. The solution then conducts electricity, which is why salt water (unlike pure water) completes a circuit, and why ionic solutions can be electrolysed.

Example 2. Comparing melting points by charge. Sodium chloride (ions $+1$ and $-1$) melts at a high temperature, but magnesium oxide (ions $+2$ and $-2$) melts even higher because the doubled charges attract far more strongly. This trend lets you predict relative melting points from the ionic charges alone.

Try this

Q1. State what happens to electrons when an ionic bond forms between a metal and a non-metal. [1 mark]

• Cue. Electrons are transferred from the metal to the non-metal, forming positive and negative ions.

Q2. Give the formula of the compound formed between calcium ($\text{Ca}^{2+}$) and chloride ($\text{Cl}^-$). [2 marks]

• Cue. One $\text{Ca}^{2+}$ needs two $\text{Cl}^-$ to balance the charge, so the formula is $\text{CaCl}_2$.

Q3. Explain why magnesium oxide does not conduct electricity when solid but does when molten. [2 marks]

• Cue. When solid the ions are fixed in the lattice; when molten the ions are free to move and carry charge, so it conducts.