Covalent bonding: 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

The idea

A covalent bond is a shared pair of electrons between two non-metal atoms. Chemists often draw each shared pair as a single straight line between the atom symbols, so H₂ becomes H—H, and each line counts as one shared pair. In small molecules such as H₂O, CH₄ and CO₂, draw the central atom, attach each bonded atom with one line per shared pair, and add any unbonded outer-shell electrons as dots if required. In polymers, show the repeating unit by drawing the two backbone atoms with a line between them and extending bonds through brackets with n outside. In giant covalent structures such as diamond, graphite and silicon dioxide, draw a representative fragment: each carbon in diamond uses four single lines to four neighbours, while each silicon in silicon dioxide bonds to four oxygens and each oxygen to two silicons.

What earns the marks

• A single line between two atom symbols represents one shared pair of electrons, so a double bond needs two parallel lines and a triple bond needs three.
• Small molecules are drawn with the correct number of bonds per atom: carbon forms four single bonds, oxygen two, hydrogen one and nitrogen three.
• In a polymer repeating unit, the two bonds that join to the next unit are drawn passing through the brackets, with n written outside to show repetition.
• In a giant covalent fragment, every atom shown must have its full bonding, for example four lines around each carbon in diamond and two lines around each oxygen in silicon dioxide.
• Unbonded outer-shell electrons may be shown as dots, but the line itself already represents the shared pair, so no extra dots should be placed on the line.

Common mistakes and how to put them right

• Drawing a double bond as one line.

  Put it right: The error is that one line means one shared pair, so the correction is to draw two parallel lines for O=O or C=O.

• Leaving a hydrogen atom with two bonds in a small molecule.

  Put it right: The error is that hydrogen can only form one single bond, so the correction is to check each hydrogen has exactly one line.

• Drawing polymer brackets without bonds crossing them.

  Put it right: The error is that the repeating unit would not connect to its neighbours, so the correction is to extend the two backbone bonds through the brackets and write n outside.

Exam tips

• Count the outer-shell electrons each atom needs before drawing, then check that every atom has the right number of lines around it.
• For polymers, draw the repeating unit only once inside the brackets and place the n clearly outside, not inside.
• For giant covalent structures, label the diagram as a fragment or part of the structure so the examiner sees you are not trying to draw the whole lattice.