Modelling molecules

Dot and cross diagrams

A dot and cross diagram can model the bonding in a simple molecule:

  • the outer shell of each atom is drawn as a circle
  • circles overlap where there is a covalent bond
  • electrons from one atom are drawn as dots, and electrons from another atom as crosses

For example, a hydrogen molecule forms when a hydrogen atom shares its outer electron with another hydrogen atom.

Dot and cross diagram of a hydrogen bondingA dot and cross diagram to model the bonding in hydrogen, H2

Drawing structures

A simple molecule can be modelled by drawing its structure. In these structures:

  • show each atom by its element symbol
  • show each covalent bond as a straight line
Two letter Hs connected by a line

For example, a hydrogen molecule has two H atoms and one covalent bond. The diagram shows its structure.

Some examples of models

Table of models of hydrogen chloride, water and methane