Ionic bonding and dot and cross diagrams

How the electrons move

Take sodium chloride. A sodium atom has 11 electrons, arranged 2,8,1. That means 2 in the first shell, 8 in the second and 1 in the outer shell. A chlorine atom has 17 electrons, arranged 2,8,7.

The sodium atom passes its one outer electron to the chlorine atom. Sodium becomes Na⁺, with 2,8. Chlorine becomes Cl⁻, with 2,8,8. Both ions now have a full outer shell, the same arrangement as a noble gas in Group 0.

The nucleus does not change. Sodium keeps its 11 protons and now has 10 electrons, so its charge is +1. An ion forms by losing or gaining electrons, never protons.

How to draw a dot and cross diagram

1. Write the electron arrangement of each atom.
2. Draw the outer shell of each atom. Use dots for one atom's electrons and crosses for the other's.
3. Move electrons from the metal to the non-metal until the non-metal's outer shell is full.
4. Redraw each ion and write its charge at the top right, such as Na⁺ or Cl⁻.
5. Check that the charges add up to zero.

The diagram builder lets you pick a pair of elements and move the electrons across one at a time. It shows when both ions are complete.

Working out the charge from the group

For the groups GCSE uses, the group number tells you the charge:

• Group 1 metals lose 1 electron and form 1+ ions, such as Na⁺.
• Group 2 metals lose 2 electrons and form 2+ ions, such as Mg²⁺.
• Group 6 non-metals gain 2 electrons and form 2− ions, such as O²⁻.
• Group 7 non-metals gain 1 electron and form 1− ions, such as Cl⁻.

Find each group on the periodic table. The charges then give you the formula. Magnesium chloride needs two Cl⁻ ions to cancel one Mg²⁺ ion, so its formula is MgCl₂. You need formulae like this before you start balancing equations.

Properties of ionic compounds

An ionic compound is a giant lattice, a regular pattern of ions repeated over and over. Strong forces of attraction act in every direction between the positive and negative ions. Breaking all those bonds takes a lot of energy, so ionic compounds have high melting and boiling points.

A solid ionic compound does not conduct electricity, because its ions are held in place. Once it melts or dissolves in water, the ions can move and carry charge. OCR Twenty First Century sets this out for Group 1 halides, such as sodium chloride.

What each board asks

AQA wants dot and cross diagrams for compounds of Group 1 or 2 metals with Group 6 or 7 non-metals. It also asks about the limits of these diagrams and of ball and stick models. Edexcel keeps ion formation to the same four groups. OCR Gateway asks for dot and cross diagrams of simple ionic substances made of two elements. OCR Twenty First Century builds the topic around Group 1 metals joining Group 7 non-metals.

Ionic bonding starts at GCSE. The KS3 curriculum covers atoms, elements and compounds, and leaves bonding for later.