Covalent bonding and simple molecules · GCSE Chemistry

Covalent bonds

A covalent bond is a shared pair of electrons between two atoms. Covalent bonds are strong.
Covalent bonding happens in non-metal elements and compounds.
Dot and cross diagrams show the shared pairs: for example, in H₂O the oxygen shares one pair with each hydrogen.
Some atoms share two pairs (a double bond), as in O₂ and CO₂.

Hydrogen (H₂), chlorine (Cl₂), oxygen (O₂) and nitrogen (N₂).
Hydrogen chloride (HCl), water (H₂O), ammonia (NH₃) and methane (CH₄).
The number of bonds an atom forms usually equals the number of electrons it needs to fill its outer shell: H forms 1, O forms 2, N forms 3 and C forms 4.

Substances made of small molecules are usually gases or liquids with low melting and boiling points.
They have weak intermolecular forces between the molecules. It is these forces, not the covalent bonds, that are overcome when the substance melts or boils.
Intermolecular forces increase with the size of the molecules, so larger molecules have higher melting and boiling points.
They do not conduct electricity, because the molecules have no overall electric charge.

Polymers are very large molecules. Atoms in each polymer chain are joined by strong covalent bonds.
The intermolecular forces between polymer molecules are relatively strong, so polymers are solids at room temperature.
Example: poly(ethene) is made from many ethene molecules.

Covalent bond: A shared pair of electrons between two atoms.
Molecule: Two or more atoms joined by covalent bonds.
Double bond: Two shared pairs of electrons between two atoms.
Intermolecular forces: Weak forces of attraction between molecules.
Simple molecular substance: A substance made of small molecules, such as water or methane.
Polymer: A very large molecule made of many repeating units joined by covalent bonds.
Molecular formula: The number of each type of atom in a molecule, for example H₂O.