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 to show the formation of a sodium ion

Diagram showing the formation of the sodium ion

Diagram to show the formation of a chloride 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 & Cross Diagrams

• These show the arrangement of the electrons in an ionic compound
• The electrons are shown as dots and crosses
• The charge of the ion is spread evenly which is shown by using brackets
• The charge on each ion is written at the top right-hand corner

Dot and cross diagram of sodium chloride

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
• If there are more than two atoms, then hollow circles or other symbols / colours may be used to make it clear
• The electrons from each atom should be represented by using solid dots and crosses
• 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

Forming the ionic bond in sodium chloride

Diagram representing how the ionic bond in sodium chloride is formed

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

Ionic bonding in magnesium oxide

Dot-and-cross diagram of magnesium oxide

Formula of ionic compounds

• 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