3.1.3 (a) Halogens' melting and boiling points

Syllabus

(a) existence of halogens as diatomic molecules and explanation of the trend in the boiling points of Cl₂, Br₂ and I₂, in terms of induced dipole–dipole interactions (London Forces) (see also 2.2.2 k)

What does this mean?

Prepare to be patronised again... you did this at GCSE!

All halogens exist as diatomic molecules - molecules made of two atoms, obviously.

Since all Halogens have 7 valence electrons they all need one more electron to fill their outer shell.

So they all form molecules with a single covalent bond, like Bromine (left)

Equally obviously, the molecules get bigger as you go down the group - more shells.

This makes their bond-lengths longer and the bond enthalpies (strength) weaker.

You should still recall colours and states of the Halogens at room temperature but now you'll have to add an explanation of why the states are different.

You won't have to recall the numbers but you might still be asked to predict one if you are given the others - still a GCSE question.

All Halogens form non-polar bonds - both atoms have the same electronegativity.

So the only intermolecular forces possible are van der Waals (or London Dispersion forces, if you prefer).

And the strength of van der Waals is determined by the number of electrons.

So, Iodine molecules with a total of 106 electrons are able to produce strong enough temporary and induced dipoles to be a solid at 20^oC.

Whereas Fluorine molecules with only 18 electrons, and Chlorine with only 34 electrons can't produce strong enough temporary and induced dipoles to avoid being gases.

Bromine has just enough electrons with 70 to be a volatile liquid.

Exam-style Questions

1. State and explain the trend in electronegativity and the trend in boiling point of the halogens from Fluorine to Iodine.

Trend in electronegativity........................................................................................................................................

Explanation...........................................................................................................................................................

...........................................................................................................................................................................

Trend in boiling point.............................................................................................................................................

Explanation...........................................................................................................................................................

...........................................................................................................................................................................

(Total 6 marks)

Answers

1. Trend in electronegativity decreases (1)

Explanation more shielding/radius increases (1)

attraction for bonding pair decreases (1)

Trend in boiling point increases (1)

Explanation van der Waals forces increase (1)

with molecular size/number of shells (1) 6

[6]

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