Polarity and intermolecular forces

Some atoms attract bonding electrons more than other atoms

An atom’s ability to attract the electron pair in a covalent bond isa called electronegativity.

Fluorine is the most electronegative element. Closely followed by Oxygen, Nitrogen and Chlorine. It is measured on the Pauling Scale

Covalent bonds may be polarised by differences in electronegativity

In a covalent bond the more electronegative atom will pull the bonding pair towards itself. In a polar bond the difference causes a permanent dipole. Covalent bonds of diatomic gases are non-polar.

In polar molecules, charge is arranged unevenly

If a molecule has a symmetric arrangement of dipoles=, they cancel each other out meaning the molecule is non-polar. But if they don’t cancel one another out ,they are polar.

There’s a gradual transition from ionic to covalent bonding

Only bonds between atoms of a single element can be purely covalent. This is the because the electronegativity difference is zero. And only a few molecules are completely ionic, most are in between.

Intermolecular forces are very weak

1) Induced dipole-dipole or London forces

2) Permanent dipole-dipole interactions

3) Hydrogen bonding

Induced dipole-dipole forces are found between all atoms and molecules

They cause all atoms and molecules to be attracted to each other.

Electrons move in clouds and they at any one time they will be more on one side of an atom than another, this temporary dipole can cause another in the opposite direction on a neighbouring atom, the two dipoles attract one another.

Stronger induced dipole-dipole forces means higher boiling points

Larger molecules have larger electron clouds, meaning stronger induced dipole-dipole forces. Also molecules with larger surface area are stronger because more exposed electron cloud.

Polar molecules form permanent dipole-dipole interactions

The partial positive & negative charges on polar molecules cause weak electrostatic forces between molecules called permanent dipole-dipole interactions. Happen in addition to induced.

Hydrogen bonding is the strongest intermolecular force

Can only happen when hydrogen is covalently bonded to Fluorine, Nitrogen or Oxygen. H has a high charge density because it’s so small and the other 3 are very electronegative. The bond is so polarised that the H of one molecule forms a weak bond with the lone pair of electrons on the F,N,O of another molecule.

Hydrogen bonds with water

They are soluble in water and have high bp’s and freezing points.

When ice is melted, hydrogen bonds are broken, so ice has more of these bonds than liquid water. Since the hydrogen bonds are long, this makes ice less dense than water

Intermolecular forces explain the trends in boiling points

Generally the main factor of bp is the strength of induced dipole-dipole forces (unless h bonds can be formed).

This is why HI has a higher bp than HCl because even though permanent dipole-dipole interactions are decreasing, the number of electrons in the molecule means that induced dipole-dipole forces also increase

But if 2 molecules have similar number of electrons the induced will be similar. So if one substance is more polar than the other, it will have stronger permanent so higher bp.

Intermolecular forces explain the behaviour of Simple Covalent Compounds (SCC)

SCC — low MP/BP because intermolecular forces are weak

Polar molecules — water is polar so tends to dissolve polar molecules.

SCC — don’t conduct electricity because overall covalent molecules are uncharged

Covalent bonds DON’T break during MELTING and BOILING

To melt or boil a SCC you only have to overcome intermolecular forces. You don’t need to break the much stronger covalent bonds that hold the atoms together