IM Forces Notes 11.4

Dispersion (London) Forces

Think of the electron cloud being agitated • • Most common type of intermolecular force.

Caused by temporary induced- dipoles formed in adjacent molecules.

• All molecules have dispersion forces, the strength depends on 2 factors: – *The # of electrons in the molecule

F 2 Cl 2 Br 2 I 2 -188 -34 59 184

• As molar mass increases, dispersion forces become stronger, the boiling pt of non-polar molecules increases.

Dipole Forces

• Electrically attractive forces between + and – end of adjacent polar molecules. • • Boiling points of – N 2 -196C – O 2 -183C NO -151C • The Nitrogen Monoxide is slightly polar and therefore has weak dipole forces. This explains the relatively higher melting points.

Hydrogen Bonding

• • Unusually strong DIPOLE forces.

This is due to the very small Hydrogen atom’s Electronegativity difference with: F fluorine O oxygen N nitrogen • The strongest of the ‘weak forces’ Water H 2 O bp = 100 C, H 2 S -61C