Students should be able to represent: a covalent bond using a line a…

The idea

Representation carries its own marks. An ordinary covalent bond is drawn as a straight line between the two atoms, one line for each shared pair. A co-ordinate bond is drawn as an arrow whose head points from the atom supplying the lone pair towards the atom receiving it, so in the ammonium ion the arrow runs from nitrogen to the added hydrogen. Do not confuse this with a curly arrow, which is reserved for showing the movement of an electron pair in a mechanism and must start at a lone pair or a bond. A displayed formula must show every atom and every bond, and an ion needs its charge written outside a square bracket enclosing the whole species.

What earns the marks

• A straight line drawn for each shared pair in an ordinary covalent bond.
• An arrow drawn for the co-ordinate bond.
• The arrowhead pointing from the donor atom towards the acceptor atom.
• The overall charge shown outside a bracket enclosing the whole ion.

Common mistakes and how to put them right

• Drawing a curly arrow, which means the movement of an electron pair, in place of the straight dative arrow.

• Pointing the arrow from the acceptor back to the donor.

• Drawing four plain lines in the ammonium ion so the dative bond cannot be identified.

• Leaving the charge off the ion or placing it on a single atom rather than outside the bracket.

Exam tips

• Read the question: if it says show the co-ordinate bond, a plain line will not score.
• In a displayed formula show all bonds and all atoms, including every hydrogen.
• Keep diagrams unambiguous; a structure open to two readings earns nothing.