Covalent molecules, dot and cross diagrams and models

In covalent bonds, atoms share pairs of electrons. Dot and cross diagrams show which electrons are shared, but every model of a molecule has limits.

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Shared pairs

A covalent bond is a shared pair of electrons between two non-metal atoms. Each atom usually ends up with a full outer shell.
Covalent bonding makes molecules. Atoms and small molecules are tiny: about 1 × 10-10 m across.
MoleculeBonds
Hydrogen, H21 single bond
Hydrogen chloride, HCl1 single bond
Water, H2O2 single bonds (O-H)
Methane, CH44 single bonds (C-H)
Oxygen, O21 double bond (2 shared pairs)
Carbon dioxide, CO22 double bonds (4 shared pairs)
C H H H H • carbon electron × hydrogen electron
Dot and cross diagram for methane, CH₄: four shared pairs (8 shared electrons).

Drawing dot and cross diagrams

Use dots for the electrons of one atom and crosses for the other. Put the shared pairs in the overlap between the atoms.
Only the outer shell electrons need to be shown.
In water, oxygen (6 outer electrons) shares one pair with each hydrogen, giving oxygen 8 outer electrons and each hydrogen 2.
Poly(ethene) is a simple polymer: a very long chain of carbon atoms joined by single covalent bonds, each with two hydrogen atoms.
O H H shared pair • oxygen electron × hydrogen electron
Dot and cross diagram for water, H₂O: two shared pairs (outer shells only).

Models and their limits

Dot and cross: shows which atom each electron came from, but not the 3D shape.
Displayed formula (H-O-H): shows bonds, but not the shape or the electrons.
Ball and stick: shows 3D shape, but bonds are not really sticks, and atoms are not really coloured balls; sizes are not to scale.
Space-filling: shows relative sizes, but hides the bonds.
No single model shows everything.
Worked example

How many electrons are shared in total in a molecule of methane, CH4?

  1. Carbon forms 4 single covalent bonds, one to each hydrogen.
  2. Each single bond is one shared pair = 2 electrons.
  3. 4 × 2 = 8 electrons.

Answer: 8 electrons (4 shared pairs)

Key idea

A covalent bond is a shared pair of electrons. H2, HCl, H2O and CH4 have single bonds; O2 has a double bond and CO2 has two double bonds. Atoms and small molecules are about 10-10 m across. Every model has limitations.

Key words
Covalent bond
A shared pair of electrons between two non-metal atoms, held by the attraction of both nuclei.
Atom
The smallest particle of an element that can take part in a chemical reaction.
Polymer
A very large molecule made of many repeating units joined together.
Electron
A tiny negative particle (relative charge -1) found in shells around the nucleus.
Displayed formula
A formula that shows every atom and every bond in a molecule.

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Extra practice

2 more questions

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  1. How many shared pairs of electrons are there in a molecule of nitrogen, N2?

    Show the answer

    Answer: 3

    N2 has a triple bond: 3 shared pairs.

  2. How many electrons are shared in total in a molecule of methane, CH4?

    Show the answer

    Answer: 8

    Four single bonds, each a shared pair of 2 electrons: 8 electrons.

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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