Intermolecular Bonds

Permanent Dipoles

Formed when two atoms in a bond have substantially different electronegativites
The more electronegative atom attracts the electrons more, so it becomes slightly negative.
The other atom in the bond becomes slightly positive due to the absence of electrons
This forms a polar bond.
This can then go on to attract other permanent dipoles, forming a permanent dipole - permanent dipole intermolecular bond.
A dipole in a molecule could also induce a dipole in a neighbouring molecule.
The delta negative part of the molecule repels the electrons on the neighbouring molecule away, so causing it to be slightly positive.
The slightly positive end of one molecule and slighly negative end of the other molecule are then attracted towards each other.
This forms a permanent dipole - induced dipole bond.

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Electrons in a molecule randomly move around the molecule.
At one point they may be unevenly distributed - one side may be more concentrated with electrons than the other
- this causes that side to be slightly negative and the other side to be slightly positive.
At this instant, this creates a dipole in the molecule

This can then induce a dipole in a neighbouring molecule (the delta negative side repels electrons on the neighbouring molecule so making it slightly positive)
These two molecules then attract each other, forming an instantaneous dipole - induced dipole bond.

Larger atoms form stronger instantaneous dipole - induced dipole bonds
They have more electrons so can create more positive/negative areas of charge
The induced dipole is therefore also stronger
This leads to greater attraction between the molecules, so take more energy to break.

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Forms when a hydrogen is attached to a small and electronegative N/O/F atom, which draws the bonding electrons towards itself (due to it's electronegativity), meaning it becomes quite negative (a high delta negative charge)
This causes the hydrogen to become very positive.
The delta positive hydrogen atom is then attracted to a lone pair on a delta negative N/O/F atom attached to a neighbouring molecule.

Hydrogen bonding is the strongest type of intermolecular bonding (apart from giant covalent network/ionic lattice), and so molecules with hydrogen bonds will have high boiling points.
Molecules which can form hydrogen bonds with water are usually soluble
- too few OH groups = cannot form enough hydrogen bonds to break the molecules apart
- too many OH groups = water cannot break so many hydrogen bonds apart so it is insoluble.

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