Covalent bonding

There are three types of strong chemical bonds: ionic, covalent and metallic.

For ionic bonding the particles are oppositely charged ions. For covalent bonding the particles are atoms which share pairs of electrons. For metallic bonding the particles are atoms which share delocalised electrons.

You should be able to explain chemical bonding in terms of electrostatic forces and the transfer or sharing of electrons.

All three types of bond come down to the same thing: negative electrons attracted to positive nuclei. What differs is what happens to the outer electrons. In ionic bonding they are transferred from one atom to another, and the oppositely charged ions then attract each other. In covalent bonding they are shared between two nuclei, both of which attract the shared pair. In metallic bonding they are given up to the whole structure, and every positive ion is attracted to the sea of electrons around it. Every one of those attractions is electrostatic — that is the word an examiner is looking for.

When atoms share pairs of electrons, they form covalent bonds. These bonds between atoms are strong.

Covalently bonded substances may consist of small molecules. You should be able to recognise common substances that consist of small molecules from their chemical formula — a formula built from a few non-metal atoms, such as H₂O, CO₂ or NH₃, is a small molecule.

Some covalently bonded substances have very large molecules, such as polymers. Some have giant covalent structures, such as diamond and silicon dioxide.

Polymers can be represented in a form where a repeating unit is written in brackets with a subscript n, where n is a large number. That notation says the same short unit repeats along a very long chain, rather than listing thousands of atoms.

You should be able to recognise substances as small molecules, polymers or giant structures from diagrams showing their bonding. The test is what the diagram shows at its edges: a small molecule is complete, a polymer has a bracketed unit with bonds running out of both sides, and a giant covalent structure has bonds continuing off the edge of the picture in every direction.

You should be able to:

• draw dot and cross diagrams for the molecules of hydrogen, chlorine, oxygen, nitrogen, hydrogen chloride, water, ammonia and methane • represent the covalent bonds in small molecules, in the repeating units of polymers, and in part of giant covalent structures, using a line to represent a single bond • describe the limitations of using dot and cross, ball and stick, two-dimensional and three-dimensional diagrams to represent molecules or giant structures • deduce the molecular formula of a substance from a given model or diagram in these forms showing the atoms and bonds in the molecule.