4CH1

Ionic Bonding

Principles of Chemistry · 2 question types

Practise

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 9% of your exam marks.

stable
Medium
Stable9%

Ion formation, lattice structures and properties of ionic compounds tested consistently.

  • A dot-and-cross diagram shows the transfer of electrons when atoms become ions and an ionic bond forms
  • Conventions to follow:
    • Use dots for one atom's electrons and crosses for the other atom's electrons
    • Show only the outer-shell electrons (Edexcel does not require inner shells)
    • Place each finished ion inside square brackets with its charge written as a superscript at the top-right, outside the brackets
    • Any transferred electron keeps the symbol of its original atom (so a cross from chlorine onto the sodium ion is still drawn as a cross)

Sodium chloride: Na⁺ and Cl⁻

  • Sodium (Group 1) has 1 outer electron and loses it
  • Chlorine (Group 7) has 7 outer electrons and gains 1 to make 8
  • One electron transfers from Na to Cl
  • The Na atom becomes Na⁺ (configuration 2,8); the Cl atom becomes Cl⁻ (configuration 2,8,8)
  • Formula: NaCl
Dot-and-cross diagram of sodium chloride: one electron transfers from a sodium atom to a chlorine atom, forming Na⁺ and Cl⁻ ions each with a full outer shell
Source: Ionic bonding diagrams by Save My Exams

Magnesium oxide: Mg²⁺ and O²⁻

  • Magnesium (Group 2) has 2 outer electrons and loses both
  • Oxygen (Group 6) has 6 outer electrons and gains 2 to make 8
  • Two electrons transfer from Mg to O
  • The Mg atom becomes Mg²⁺; the O atom becomes O²⁻
  • Formula: MgO
Dot-and-cross diagram of the Mg²⁺ and O²⁻ ions in magnesium oxide, each shown in square brackets with its charge
Source: Ionic bonding diagrams by Save My Exams

What goes in, what stays out

  • Only the outer shell of each atom is drawn (inner shells are omitted)
  • The transferred electrons still belong to their original atom and so are drawn as their original symbol on the new ion
  • Square brackets and charge superscripts go around each finished , not around the starting atoms
Exam tip

Drawing the electronic configuration of ions

What comes up: "Draw the electronic configuration of a [metal] ion and a [non-metal] ion. Show the charge on each ion." A partially completed diagram is usually provided. (3 marks)

Write (three marks): (1) The metal ion drawn correctly (e.g. Mg²⁺ with electrons 2,8, in square brackets with 2+ outside). (2) The non-metal ion drawn correctly (e.g. Cl⁻ with 8 outer electrons, in square brackets with 1− outside). (3) Both charges labelled correctly as superscripts outside the brackets.

Watch out: The mark scheme awards at most 1 mark for M1 and 1 mark for M2 if you show only the outer electrons when the question explicitly asks for the full electronic configuration — inner shells must be included when specified. Dots and crosses can be used interchangeably (ALLOW any combination).