Covalent bonding
A covalent bond is a shared pair of electrons between atoms. Non-metal atoms can share electrons to form molecules, such as water and oxygen. Some substances, such as diamond, instead have a giant network of atoms joined by covalent bonds.
Sort substances by bond type in Bonding Blitz.
How covalent bonding works
Atoms share pairs of outer-shell electrons in covalent bonds. Each shared pair counts towards the outer shell of both bonded atoms. A single bond is one shared pair; a double bond is two shared pairs and a triple bond is three.
In a water molecule, oxygen shares one pair with each of two hydrogen atoms. Each hydrogen has two electrons in its first shell, and oxygen has eight in its outer shell. Use Dot and Cross to practise reading diagrams.
Reading dot and cross diagrams
Dots and crosses distinguish electrons from different atoms. A shared pair sits between the bonded atoms. Pairs of outer-shell electrons that are not shared in a bond are called lone pairs. The diagram shows outer electrons, not the exact shape or size of a molecule.
The formula says how many atoms of each element are in a molecule. The relative formula mass guide shows how to add their relative atomic masses.
Simple molecules and giant structures
Simple molecular substances have strong covalent bonds within each molecule and weak forces between molecules. Melting or boiling overcomes forces between molecules, so these substances usually have low melting and boiling points. The covalent bonds within molecules do not need to break.
Diamond, graphite and silicon dioxide have giant covalent structures. Many atoms are joined by strong covalent bonds throughout the structure, so melting needs much more energy. In diamond each carbon bonds to four others. In graphite each carbon bonds to three others in layers. One electron from each carbon atom is delocalised, so graphite conducts electricity.
Covalent and ionic bonding
Covalent bonding involves shared electrons. In ionic bonding, electrons transfer to form oppositely charged ions, and the ions attract. Ionic substances form giant lattices rather than separate molecules.
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Common mistakes
- Saying the covalent bonds inside molecules break when a simple molecular substance melts. The weak forces between molecules are overcome.
- Calling all covalent substances molecules. Diamond and graphite are giant structures.
- Counting a shared pair for only one atom when checking its outer shell.
Try Periodic Table Challenge to revisit electron arrangements.
Questions people ask
What is a covalent bond?
A covalent bond is a shared pair of electrons between atoms.
Why do simple molecular substances have low melting points?
The forces between their molecules are weak, so relatively little energy is needed to overcome them.
What is the difference between ionic and covalent bonding?
Covalent bonds involve shared pairs of electrons. Ionic bonding is the attraction between oppositely charged ions formed by electron transfer.
Is diamond a molecule?
No. Diamond has a giant structure of carbon atoms joined by covalent bonds.