intermolecular forces

what are the 3 main types of intermolecular force in order of increasing strength?

• Van der Waals forces

• permanent dipole-dipoles

• hydrogen bonding

what are intermolecular forces?

forces that occur between molecules

describe and explain Van der Waals forces

• present in all molecular substances

• occur because e- are constantly moving around and there will be an uneven e- distribution at any given moment in time

• this causes a TEMPORARY DIPOLE w/in a molecule

• this temporary dipole induces an INDUCED DIPOLE in a neighbouring molecule

• there is then an attraction between these molecules -this is a temporary induced dipole-dipole attraction

how does increasing the size of a molecule affect bpt?

• the bigger the molecule, the greater the Van der Waal forces

• this is because bigger molecules have more e-

• ∴ greater VDWs = higher bpt

describe and explain permanent dipole-dipole attraction

• occurs between polar molecules

• polar molecules have permanent dipoles, meaning that one atom in a molecule is more electronegative than the other as the e^-s are more attracted to it

• this more electronegative atom becomes attracted to the more electropositive atom of another polar molecule

• this causes a permanent dipole-dipole attraction

  

describe and explain hydrogen bonding

• H bonded to a very electronegative atom (F, O and N)

• the polar bond between the H and N/O/F leaves the H nucleus exposed as H only has 1 e^-

• ∴ there is a strong attraction from the lone pair on the N/O/F of 1 molecule to the exposed nucleus of the other

how do we find H bonds?

H atoms bonded to a N/O/F atom

do lone pairs of e^- conduct electricity? why?

• no

• they are localised and so not free to move