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
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
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).