Dot-and-Cross Diagram
Summary: A diagram showing the outer-shell electrons of the atoms in a bond, using dots for the electrons of one atom and crosses for the electrons of the other. Tags: igcse chemistry Created: 2026-07-18
A dot-and-cross diagram is a pictorial representation of chemical bonding that shows only the outer-shell (valence) electrons of the atoms involved, drawing the electrons of one atom as dots (•) and the electrons of the other atom as crosses (×) so that the origin of each electron can be tracked. For an Ionic Bond, the diagram shows the transfer of electrons from a metal atom to a non-metal atom, with the resulting ions enclosed in square brackets carrying their charges. For a Covalent Bond, the diagram shows overlapping outer shells (or paired symbols between the atoms) in which shared pairs of electrons hold the atoms together in a Molecule. Although dots and crosses suggest the electrons are distinguishable, in reality all electrons are identical — the symbols are purely a bookkeeping device. These diagrams are central to Ions and Ionic Bonds and Covalent Bonding because they make the electron transfer or electron sharing explicit, and they allow the charges of ions and the number of bonds in molecules to be predicted from Electron Shell configurations.
Key Conventions
- Only outer-shell (valence) electrons are drawn; inner shells are usually omitted.
- Electrons from one atom are drawn as dots (•); electrons from the other atom as crosses (×).
- Electrons are drawn in pairs where possible.
- Ionic diagrams use square brackets around each ion with the charge at the top right.
- Covalent diagrams use overlapping circles, with the shared pair(s) placed in the region of overlap.
Dot-and-Cross Diagrams for Ionic Compounds
In an Ionic Bond, electrons are transferred from the metal atom to the non-metal atom so that both achieve a full outer shell like a noble gas (Group 0 Noble Gases). The diagram shows:
- The metal atom losing its outer electron(s) — often indicated with a curly arrow from the metal’s outer shell to the non-metal’s outer shell.
- The resulting positive ion drawn with an empty outer shell (or no outer shell shown), in square brackets with its charge.
- The negative ion drawn with a full outer shell, including the transferred electron(s) marked in the other symbol (e.g. a cross among dots), in square brackets with its charge.
Example: Sodium Chloride (NaCl)
Sodium (2,8,1) transfers its single outer electron to chlorine (2,8,7):
Na • ──arrow──► Cl (7 crosses)
[ Na ]⁺ [ ×× Cl ×× • ]⁻
(8 electrons: 7 crosses + 1 dot)
The sodium ion [Na]⁺ has lost its dot; the chloride ion [Cl]⁻ has eight outer electrons — seven crosses of its own plus the one dot gained from sodium.
Example: Magnesium Oxide (MgO)
Magnesium (2,8,2) transfers two electrons to oxygen (2,6):
Mg •• ──2 arrows──► O (6 crosses)
[ Mg ]²⁺ [ ×× O ×× •• ]²⁻
(8 electrons: 6 crosses + 2 dots)
Because two electrons are transferred, the ions carry 2+ and 2− charges, giving a stronger electrostatic attraction than in NaCl — one reason MgO has a much higher melting point (see Ionic Compounds and Giant Ionic Lattice).
Dot-and-Cross Diagrams for Covalent Molecules
In a Covalent Bond, atoms share pairs of electrons. The diagram is drawn as two (or more) overlapping circles, one per atom, with each shared pair shown as one dot and one cross in the overlap region. Non-bonding (lone) pairs sit on the circle away from the overlap.
| Molecule | Bonds shown | Description of diagram |
|---|---|---|
| H₂ | 1 single bond | Two overlapping circles; one dot and one cross in the overlap. Each H “sees” 2 electrons. |
| H₂O | 2 single bonds | O circle overlaps two H circles; each overlap holds one shared pair; O keeps 2 lone pairs. |
| CH₄ | 4 single bonds | Central C circle overlaps four H circles; each overlap holds one shared pair (dot + cross). |
| CO₂ | 2 double bonds | C circle overlaps each O circle; each overlap holds two shared pairs; each O keeps 2 lone pairs. |
For example, in methane the carbon atom (2,4) shares each of its four outer electrons with one hydrogen electron, so carbon achieves eight outer electrons and each hydrogen achieves two. In carbon dioxide, each carbon–oxygen overlap contains four electrons (two shared pairs), representing a double bond. Molecules such as Ammonia (NH₃, three shared pairs and one lone pair on nitrogen) follow the same rules.
Sources
- Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025) — Section 2.4 (Core 3, Supplement 6) and Section 2.5 (Core 2, Supplement 4), Cambridge Assessment International Education
Common Misconceptions
| Misconception | Reality |
|---|---|
| Dots and crosses represent two different kinds of electron. | All electrons are identical; dots and crosses only record which atom each electron came from. |
| Ionic dot-and-cross diagrams show a bond line between the ions. | There is no shared pair in ionic bonding — the diagram shows complete transfer, and the “bond” is the electrostatic attraction between oppositely charged ions. |
| Square brackets and charges are optional decoration. | Brackets and charges are required for ions; without them the diagram shows neutral atoms, not an ionic compound. |
| Every shared region contains exactly two electrons. | Double bonds (as in CO₂) have four electrons (two shared pairs) in the overlap region. |
| All shells and electrons must be drawn. | Only the outer shell is normally shown at IGCSE; inner shells are omitted for clarity. |