4.4 Intermolecular Forces

Simple molecular substances often exist as gases but (most) will condense into liquids and freeze into solids if the temperature is low enough.

Calculations show that Gravity is not a strong enough force to do this.

Therefore there must be forces of attraction even between gaseous molecules.

Very simple molecular elements like H₂, Cl₂, N₂ etc do not have permanent dipoles because the two atoms have the same electronegativity.

But they might develop temporary dipoles if electrons shift unevenly.

Example

I₂ – has many electrons loosely held.

Sometimes there will be more electrons on one side of the molecule than the other.

So, a temporary dipole occurs.

This may even happen in a single atom of Neon but with few electrons the dipole will be smaller.

1. Halogen elements only have London Dispersion forces. Suggest why the trend in melting/boiling point is upwards as we go down the group

2. Isomers have the same number of electrons. Why do their boiling points sometimes differ?

A polar molecule will attract other polar molecule in the opposite orientation.

Note.

Just because a molecule has a permanent dipole (and so has dipole-dipole forces) does not mean that it doesn’t also have London Forces!

All molecules also London Forces irrespective of other intermolecular forces they may have.

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Use your knowledge of VSEPR, shapes of molecules and polarity to account for this difference

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