Students should be able to: explain the trend in electronegativity explain…

The idea

Both explanations have to be built, not asserted. Electronegativity is the power of an atom to attract the pair of electrons in a covalent bond towards itself, which is not the same as its ability to gain an electron. So the explanation must name the increase in atomic radius and the increase in shielding, which together put the bonding pair further from, and less exposed to, the nucleus. The boiling point explanation must name the structure as simple molecular, name the intermolecular force as van der Waals or induced dipole-dipole, and then link the strength of that force to the number of electrons in each molecule. The commonest lost mark is saying that bonds are broken when a halogen boils; the second commonest is offering relative molecular mass as the cause with no mention of electrons.

What earns the marks

• Defining electronegativity as the power of an atom to attract the electron pair in a covalent bond.
• Referring to atomic radius and shielding, not only nuclear charge, in the electronegativity explanation.
• Naming the structure of the halogens as simple molecular.
• Naming van der Waals forces as the forces overcome on boiling.
• Stating that the number of electrons per molecule increases down the group.

Common mistakes and how to put them right

• Defining electronegativity as the ability to gain an electron, which describes electron affinity instead.

• Giving only nuclear charge as the reason, with no mention of distance or shielding.

• Saying molecules are broken apart into atoms when the halogen boils.

• Answering that bigger molecules have higher boiling points, with no mechanism given.

• Using the word 'stronger' without naming which force is stronger.

Exam tips

• Learn the electronegativity definition word for word; it is often worth a mark on its own.
• In a boiling point answer, write structure, then force, then reason, in that order, so no step is missed.