Covalent Bond – IGCSE Chemistry Definition

A covalent bond forms when two atoms share one or more pairs of electrons. It is the bond type found between non-metal atoms and is central to the 0620 syllabus. You must be able to define covalent bonding, draw dot-and-cross diagrams, and explain the properties of simple molecular and giant covalent substances.

The 0620 definition

A covalent bond is a shared pair of electrons between two atoms. The electrostatic attraction between the shared electrons and both nuclei holds the atoms together.

How covalent bonds form

Non-metal atoms need to gain electrons to achieve a full outer shell (a stable electron arrangement like a noble gas). Instead of transferring electrons (as in ionic bonding), they share electrons.

Each atom contributes one electron to the shared pair. A single covalent bond is one shared pair; a double bond is two shared pairs; a triple bond is three shared pairs.

Key examples

Molecule	Formula	Bond type	Shared pairs	Dot-and-cross note
Hydrogen	H₂	Single	1	Each H shares 1 electron
Chlorine	Cl₂	Single	1	Each Cl shares 1 electron to fill outer shell
Water	H₂O	Single (x2)	2	O shares one pair with each H
Oxygen	O₂	Double	2	O=O, each O shares 2 electrons
Nitrogen	N₂	Triple	3	N has triple bond, each N shares 3 electrons
Methane	CH₄	Single (x4)	4	C shares one pair with each of 4 H atoms
Carbon dioxide	CO₂	Double (x2)	4	O=C=O, double bond on each side

Properties of simple covalent (molecular) substances

• Low melting and boiling points — weak intermolecular forces between molecules (not the covalent bonds) are easily overcome
• Do not conduct electricity — no free ions or delocalised electrons
• Often insoluble in water but soluble in organic solvents

Important: the covalent bonds within molecules are strong. It is the weak intermolecular forces between molecules that break during melting and boiling.

Worked exam question

Draw a dot-and-cross diagram for a molecule of water, H₂O. Show outer electrons only. (2)

Mark scheme

Oxygen shown with 2 lone pairs and 2 bonding pairs in its outer shell [1]; each hydrogen shown with 1 shared pair (one electron from H, one from O), with the shared pair clearly between the atoms [1]

(Dots for one atom, crosses for the other. Total electrons around O = 8, around each H = 2.)

Common exam mistakes

• Confusing intermolecular forces with covalent bonds. “Weak covalent bonds” is wrong — covalent bonds are strong. The weak forces are between molecules, not within them.
• Drawing dot-and-cross diagrams without distinguishing which electron comes from which atom. Use dots for one atom and crosses for the other.
• Saying covalent substances “have no bonds” because they have low melting points. They have strong covalent bonds within molecules but weak forces between molecules.