Representing Ionic Bonds

• Ionic bonds can be represented diagrammatically using dot and cross diagrams
• These are a simple and quick way to show the formation of an ionic compound
• The electrons from each atom should be represented by using solid dots and crosses
• If there are more than two atoms, then hollow circles or other symbols / colours may be used to make it clear
• The large square brackets should encompass each atom and the charge should be in superscript and on the right-hand side, outside the brackets
• For larger atoms with more electron shells, only the valence shell needs to be drawn

Diagram representing the formation of the ionic bond in sodium chloride

Example: Sodium Chloride

• Sodium is a group 1 metal so loses one outer electron to another atom to gain a full outer shell of electrons
• A positive sodium ion with the charge 1+ is formed, Na+ 
• Chlorine is a group 7 non-metal so gains one electron to have a full outer shell of electrons
• A negative chloride ion with a charge of 1- is formed, Cl –
• The ions are then attracted to one another and held together by electrostatic forces
• The formula of the ionic compound is thus NaCl

Dot-and-cross diagram of sodium chloride

Example: Magnesium oxide

• Magnesium is a group 2 metal so will lose two outer electrons to another atom to have a full outer shell of electrons
• A positive ion with the charge 2+ is formed, Mg2+ 
• Oxygen is a group 6 non-metal so will need to gain two electrons to have a full outer shell of electrons
• Two electrons will be transferred from the outer shell of the magnesium atom to the outer shell of the oxygen atom
• Oxygen atom will gain two electrons to form a negative ion with charge 2-
• The ions are then attracted to one another and held together by electrostatic forces
• The formula of the ionic compound is thus MgO

Dot-and-cross diagram of magnesium oxide

Working out the Formulae

• You may be asked to give the formula of an ionic compound from a given diagram
• If it is a dot-and-cross diagram, then just count the number of atoms of each element
• This is then equal to the empirical formula of the compound
• If it is a 3D lattice structure, then look for how many ions are in the lattice
• Write them down and balance the charges to find the formula for the compound

Exam Tip

When describing ionic bonding:

• State how many electrons the metal loses
• State the resulting charge of the metal ion
• State how many electrons the non-metal gains
• State the resulting charge of the non-metal ion
• State that the oppositely charged ions are held together by strong forces of attraction in ionic bonding
• Careful: Sometimes you might need more than one metal ion, e.g. Na2O, or non-metal ion, e.g. MgCl2, so be sure to say how many of each ion are needed in these cases