Intermolecular forces

What are intermolecular forces

An attractive force between neighbouring molecules, these are weak compared to covalent bonds. They are caused by weak attraction forces between very small dipoles in different molecules

How are London forces formed

They are caused by an instantaneous dipole, as electrons are constantly and quickly moving around nuclei. At an instant electrons may cluster more on one side, creating a separation of charge, so electrons will repel other electrons next to the molecule, inducing a dipole. Temporary dipoles attract each other, this is the London force.

Note:

Every molecule + atom experiences London forces and this is the only attraction between non-polar molecules. They are temporary

What happens to London forces as you go down a group

As you go down a group molecular size and electron number increases this means strength increases as more electrons means there are larger instantaneous dipoles, inducing stronger attractions. Therefore, more energy needed to overcome IMF increasing BP

What elements can hydrogen bonding occur in

Oxygen, Nitrogen and Fluorine

What is hydrogen bonding

Special type of permanent dipole-dipole interaction, 1/10^th strength of covalent bond, strongest IMF, represented by dashed line - - - -

Note:

Water is polar due to its overall dipole due to H-bonds and them not cancelling out.

Why does chlorine not form H-bonds

It is too large in size making attraction harder, atomic radius too high and electronegativity spread too thin

Why is ice less dense than water

• Due to rigid H-bonds, which exert outwards, between water molecules which are long so they are spread far apart. Forms a tetrahedral shape full of holes, which decrease density on freezing

• On melting lattice collapses and molecules move closer together, liquid form is denser than solid ice as there no holes