Dot-and-cross diagrams are one of the most reliable mark-scoring questions in O-Level Chemistry. The examiner is not testing creativity. They are testing whether you can count valence electrons carefully and follow a fixed set of conventions. Once you drill the conventions, these become 2 to 3 marks you almost never drop.
Here is the exact method our tutors use, built around the Combined Science Chemistry (5076/5078) and Pure Chemistry (6092) bonding sections.
Step 1: Get the Valence Electrons Right
Everything starts with the group number in the Periodic Table. For the main groups, the group number tells you the number of outer-shell electrons.
- Group I (Na, K, Li): 1 outer electron
- Group II (Mg, Ca): 2
- Group IV (C, Si): 4
- Group V (N, P): 5
- Group VI (O, S): 6
- Group VII (Cl, F): 7
- Group 0 (Ne, Ar): 8, already full
- Hydrogen: 1, and it is full with only 2 electrons
Write the electronic configuration in the margin before you draw anything. For example, chlorine is 2,8,7 and oxygen is 2,6. Most lost marks come from miscounting at this stage, not from bad drawing.
Step 2: Decide Whether the Bonding Is Ionic or Covalent
- Metal plus non-metal, so electrons are transferred. This is ionic.
- Non-metal plus non-metal, so electrons are shared. This is covalent.
The two diagram types look completely different, so getting this wrong costs the whole question.
Step 3: Ionic Diagrams, the Non-Negotiable Rules
Take sodium chloride. Sodium (2,8,1) transfers its single outer electron to chlorine (2,8,7).
Your diagram must show:
- Square brackets around each ion, with the charge written outside the top right corner: Na⁺ and Cl⁻.
- The correct charges, matching the group. Group I forms 1+, Group II forms 2+, Group VI forms 2−, Group VII forms 1−.
- Different symbols for the two sets of electrons. If you drew sodium's electron as a cross, that same cross must now appear in the chlorine outer shell, surrounded by chlorine's 7 dots. That single cross is what proves transfer happened.
- The right number of ions. Magnesium chloride is one Mg²⁺ and two Cl⁻ ions, because magnesium donates two electrons to two separate chlorine atoms.
A cation like Na⁺ has an empty outer shell. You can either draw the inner shells (2,8) or simply draw an empty bracket labelled Na⁺. Both are accepted, but be consistent, and if the question says "show only the outer shell electrons", follow that instruction exactly.
Step 4: Covalent Diagrams, Count to Eight
For covalent molecules, atoms overlap and each shared pair sits in the overlap region, drawn as one dot and one cross together.
Worked examples worth memorising:
- HCl: one shared pair, plus 3 lone pairs on chlorine. Chlorine ends with 8, hydrogen with 2.
- H₂O: two shared pairs, plus 2 lone pairs on oxygen.
- NH₃: three shared pairs, plus 1 lone pair on nitrogen.
- CH₄: four shared pairs, no lone pairs. Carbon reaches 8, each hydrogen reaches 2.
- CO₂: two double bonds, so two shared pairs between carbon and each oxygen, plus 2 lone pairs on each oxygen.
- N₂: a triple bond, three shared pairs, plus 1 lone pair on each nitrogen atom.
- Cl₂: one shared pair, plus 3 lone pairs on each chlorine.
Always finish by circling each atom mentally and counting: 8 for everything except hydrogen, which needs 2.
Common Mistakes That Cost Marks
- Forgetting lone pairs. Examiners count every electron on the paper.
- Using dots for both atoms in a covalent diagram, so the shared pair is not clearly shared.
- Missing brackets or charges on ions.
- Drawing full shells for ionic compounds and then also drawing them overlapping. Ions do not overlap, they sit side by side.
- Drawing lines for bonds when the question asks for a dot-and-cross diagram.
The Written Half of the Mark
Many questions pair the diagram with an explanation. Use mark-scheme phrasing:
- Ionic: "Electrons are transferred from the metal atom to the non-metal atom, forming oppositely charged ions held together by strong electrostatic attraction."
- Covalent: "Atoms share pairs of electrons so that each atom achieves a full outer shell, or noble gas configuration."
Avoid saying atoms "want" or "need" electrons. Say they achieve a stable, full outer shell.
A 30-Second Self-Check Before Moving On
- Did I write the electronic configurations first?
- Ionic or covalent, and does my diagram style match?
- Brackets and charges on all ions?
- Dots and crosses clearly distinguishable?
- Every atom at 8 electrons, hydrogen at 2?
Drill five molecules and three ionic compounds a night for a week and this topic becomes automatic marks in Paper 2.
Want a tutor who recently sat these papers to check your diagrams line by line? Start a free trial class on WhatsApp and see how quickly this clicks.