Ionic bonding as the transfer of electrons between metals and non-metals, the formation of positive and negative ions, dot and cross diagrams, ionic formulae, and the giant ionic lattice.

What this dot point is asking

OCR wants you to explain ionic bonding as the transfer of electrons from a metal to a non-metal, describe the ions that form, draw dot and cross diagrams, work out ionic formulae from the charges, and describe the giant ionic lattice structure. Ionic bonding is one of the three bond types and underpins the properties of salts.

How ionic bonds form

When ions form, each atom gains a full outer shell like the nearest noble gas, which is stable. For example, in sodium chloride, sodium ($2.8.1$) loses one electron to become $\text{Na}^+$ ($2.8$), and chlorine ($2.8.7$) gains it to become $\text{Cl}^-$ ($2.8.8$). The number of electrons transferred is set by the group: Group 1 metals form $1+$ ions, Group 2 form $2+$, Group 6 non-metals form $2-$ and Group 7 form $1-$.

Dot and cross diagrams

A dot and cross diagram shows where the electrons in the outer shells come from, using dots for one atom's electrons and crosses for the other's. For ionic compounds you draw the ions with their charges in square brackets, showing the outer shell of each ion after transfer.

Working out ionic formulae

To find the formula of an ionic compound, the total positive charge must equal the total negative charge so the compound is neutral overall:

1. Write the charge of each ion (from its group or a given list).
2. Find the smallest numbers of each ion that make the charges balance.
3. Write the formula with those numbers as subscripts.

For example, calcium ($\text{Ca}^{2+}$) and chloride ($\text{Cl}^-$) need two chlorides to balance one calcium, giving $\text{CaCl}_2$. Aluminium ($\text{Al}^{3+}$) and oxide ($\text{O}^{2-}$) balance as $\text{Al}_2\text{O}_3$ (total $6+$ and $6-$).

The giant ionic lattice

The formula of an ionic compound (such as $\text{NaCl}$) gives the ratio of the ions in the lattice, not a separate molecule. There are no individual molecules in an ionic solid.

Properties that follow from the structure

Because of the giant lattice held by strong electrostatic forces, ionic compounds:

• have high melting and boiling points;
• conduct electricity when molten or in solution (the ions are free to move) but not when solid (the ions are locked in place);
• are often soluble in water.