1. Forces acting between molecules (aka intermolecular forces)

2. Aims- To know that there are 3 main types of intermolecular forces- Van der Waals’ forces Van der Waals’ forces Permanent dipole-permanent dipole attractions Hydrogen bonding To be able to recognise molecules where these types of forces occur

3. Van de Waals forces Also known as induced dipole-induced dipole attractions Takes place when electron clouds in atoms (or molecules) are not evenly spread- Note- the animation is a bit misleading the charges formed are only slight ie δ + or δ -

4. This results in attractive forces- The size of these forces depends on the number of electrons in the atom or molecule – bigger molecules have more electrons and therefore the attraction is greater

5. Permanent dipole-permanent dipole attractions Which of the molecules below have permanent dipoles? CH 4 HCl Cl 2 CO 2 HI CHCl 3 These forces are stronger than van der waals forces Forces of attraction between oppositely charged permanent dipoles Increase with size of dipole

6. Hydrogen Bonds The hydrogen is attached directly to one of the most electronegative elements (N,F or O), causing the hydrogen to acquire a significant amount of positive charge. Each of the elements to which the hydrogen is attached is not only significantly negative, but also has at least one lone pair.

7. Three main examples Hydrogen bonds are significantly stronger dipole-dipole attractions-

8. Hydrogen bonds in biological molecules-