AQA GCSE Chemistry

Revision Notes

2.1.2 Ionic Bonding

Formation of Ions

  • 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

Formation of positively charged Sodium ion1

Diagram showing the formation of the sodium ion

Formation-of-negatively-charged-Chloride-ion1, IGCSE & GCSE Chemistry revision notes

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

Oppositely charged ions attraction due to electrostatic attraction, IGCSE & GCSE Chemistry revision notes

Dot and cross diagram of sodium chloride

Exam Tip

The number of electrons that an atom gains or loses is the same as the charge.
For example, if a magnesium atom loses 2 electrons, then the charge will be +2, if a bromine atom gains 1 electron then the charge will be -1.

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

Ionic bonding – Sodium Chloride, IGCSE & GCSE Chemistry revision notes

Diagram representing the formation of the ionic bond in sodium chloride

Example: Sodium Chloride

  • Sodium is a group 1 metal so will lose one outer electron to another atom to gain a full outer shell of electrons
  • A positive sodium ion with the charge +1 is formed
  • Chlorine is a group 7 non-metal so will need to gain one electron to have a full outer shell of electrons
  • One electron will be transferred from the outer shell of the sodium atom to the outer shell of the chlorine atom forming a positive sodium ion with a +1 charge
  • The chlorine atom will gain an electron to form a negatively charged chloride ion with a charge of -1
  • The ions are then attracted to one another and held together by electrostatic forces
  • The formula of the ionic compound is thus NaCl

Oppositely charged ions attraction due to electrostatic attraction, IGCSE & GCSE Chemistry revision notes

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
  • 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

Magnesium Oxide dot & cross diagram, IGCSE & GCSE Chemistry revision notes

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 writing about ions, we use the notation 1-, 2+ etc. to describe the charge of the ion, with the number first followed by the sign (+/-). It is incorrect to write them the other way around as this refers to the oxidation state, not the charge.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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