Covalent Bonds: Dot & Cross Diagrams

Lesson 1 of 2

Diatomic molecules

Diatomic molecules contain exactly two atoms, and their dot-and-cross diagrams are the simplest covalent drawings. Hydrogen shares one pair, chlorine shares one pair, oxygen shares two pairs and nitrogen shares three. Hydrogen chloride follows the hydrogen pattern, with one pair shared between a hydrogen atom and a halogen atom.

What you know already

From the earlier note in Covalent Bonding you know that a covalent bond is a shared pair of electrons between two non-metal atoms. This note puts that idea on paper: dot-and-cross drawings for the small molecules the exam names, from hydrogen and the halogens through water, ammonia and carbon dioxide, up to methane and its two-carbon relatives.

Key points of understanding

• A diatomic molecule contains exactly two atoms, such as H2, Cl2, O2, N2 or HCl.
• Hydrogen and the halogens form single covalent bonds, so one pair is shared in each of H2 and Cl2.
• Oxygen shares two pairs, giving a double bond, and nitrogen shares three pairs, giving a triple bond.
• Hydrogen chloride shares one pair between the hydrogen atom and the halogen atom.
• Each shared pair in the drawing uses one electron from each atom, keeping the donor symbols visible.