Ionic Bonding (AQA GCSE Chemistry): Revision Note

Formation of Ions

What is ionic bonding?

• Ionic bonding is the strong electrostatic force of attraction between oppositely charged ions

• It is formed when a metal atom transfers one or more electrons to a non-metal atom

• This transfer results in the formation of a positive ion (cation) from the metal atom and a negative ion (anion) from the non-metal atom

How do ions form?

• 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

• The charge on the ion is related to its group number:

  • Group 1 metals form 1+ ions

  • Group 2 metals form 2+ ions

  • Group 6 non-metals form 2− ions

  • Group 7 non-metals form 1− ions

Positive ions

• Positive ions are called cations and form when atoms lose electrons

  • This means that they have more protons than electrons

• Metals typically lose electrons to form positive ions

  • For example, sodium losing one electron to form a sodium ion, Na⁺:

Negative ions

• Negative ions are called anions and form when atoms gain electrons

  • This means that they have more electrons than protons

• Non-metals typically gain electrons to form negative ions

  • For example, chlorine gaining one electron to form a chloride ion, Cl^-:

• The positive and negative charges are held together by the strong electrostatic forces of attraction between oppositely charged ions

• An ionic compound is a giant structure of ions

  • The electrostatic forces act in all directions throughout the lattice and hold the compound 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

Representing ionic bonding

• Ionic bonds can be represented diagrammatically using dot and cross diagrams

• If there are more than two atoms, hollow circles or other symbols may be used to distinguish between them

• Electrons from each atom are represented using solid dots and crosses

• Large square brackets should encompass each ion

• The charge is in superscript outside the brackets at the top-right

• For larger atoms with more electron shells, only the valence shell needs to be drawn

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 attract each other through electrostatic forces

• The formula of the ionic compound is NaCl

Example: Magnesium oxide

• Magnesium is a Group 2 metal so loses two outer electrons 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 gains two electrons to have a full outer shell of electrons

  • Oxygen atom will gain two electrons to form a negative ion with charge 2-

• The ions attract each other through electrostatic forces

• The formula of the ionic compound is MgO

Ionic bonding in magnesium oxide

Formula of ionic compounds

• If given a dot-and-cross diagram, count the number of atoms of each element

  • This gives the empirical formula

• If given a 3D lattice structure, identify the ions present and balance the charges to find the formula