Ionic Bonding (AQA GCSE Chemistry: Combined Science)

• An ion is an electrically charged atom or group of atoms formed by the loss or gain of electrons
• This loss or gain of electrons takes place to obtain a full outer shell of electrons
• The electronic structure of ions of elements in groups 1, 2, 6 and 7 will be the same as that of a noble gas - such as helium, neon, and argon
• Negative ions are called anions and form when atoms gain electrons, meaning they have more electrons than protons
• Positive ions are called cations and form when atoms lose electrons, meaning they have more protons than electrons
• All metals lose electrons to other atoms to become positively charged ions
• All non-metals gain electrons from other atoms to become negatively charged ions

Diagram showing the formation of the sodium ion

Diagram showing the formation of the chloride ion

• The positive and negative charges are held together by the strong electrostatic forces of attraction between oppositely charged ions
• This is what holds ionic compounds together

Dot and cross diagram of sodium chloride

• 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