Dot-and-Cross Diagram – IGCSE Chemistry Definition

A dot-and-cross diagram is a representation of chemical bonding that shows the outer-shell electrons of the atoms involved. Electrons from one atom are drawn as dots and electrons from the other atom are drawn as crosses. This makes it clear which atom each electron originally belonged to and how the electrons are shared (covalent bonding) or transferred (ionic bonding).

Covalent dot-and-cross diagrams

For covalent bonds, the shared pair of electrons is shown in the overlap region between the two atoms. Each atom contributes one electron to the shared pair. For example, in hydrogen chloride (HCl), the hydrogen atom contributes one electron (shown as a dot) and the chlorine atom contributes one electron (shown as a cross). The shared pair sits between the two atoms, and the remaining outer electrons of chlorine are shown as lone pairs.

Water (H₂O) shows two shared pairs, one between oxygen and each hydrogen. Oxygen also has two lone pairs. Methane (CH₄) shows four shared pairs, with carbon sharing one pair with each of the four hydrogen atoms.

Ionic dot-and-cross diagrams

For ionic bonding, the diagram shows the electron transfer. The metal atom loses its outer electrons, and they are shown on the non-metal atom. Each ion is drawn inside square brackets with the charge written outside. For sodium chloride, the sodium atom’s single outer electron (dot) is transferred to the chlorine atom, which gains it alongside its own seven outer electrons (crosses). The resulting ions are [Na]⁺ and [Cl]⁻ with the transferred electron clearly visible.

Exam context

Dot-and-cross diagrams appear frequently on Paper 4. Common molecules asked about include H₂, HCl, H₂O, NH₃, CH₄, O₂, CO₂, and Cl₂ for covalent bonding, and NaCl, MgCl₂, MgO, and CaF₂ for ionic bonding. Marks are awarded for correct number of outer electrons, correct positioning of shared/transferred electrons, and correct charges on ions.

Worked exam question

Draw a dot-and-cross diagram to show the bonding in magnesium oxide, MgO. Show outer electrons only. (2 marks)

Magnesium (2, 8, 2) transfers its 2 outer electrons to oxygen (2, 6). Show [Mg]²⁺ with no outer electrons and [O]²⁻ with 8 outer electrons (6 original crosses + 2 transferred dots) in square brackets [1+1].

Common mistakes

Forgetting square brackets and charges on ionic diagrams costs marks. Showing inner-shell electrons when only outer shells are asked for wastes time. Confusing dots and crosses (using the same symbol for both atoms) removes the whole point of the diagram. For covalent molecules, a common error is drawing too many or too few lone pairs on atoms like oxygen or nitrogen.