1. Read up on Valence Shell Electron Pair Repulsion Theory (VSEPR).
2. Think about what causes London Forces. Larger molecules have more electrons usually.
3. A number of polyatomic ions start off as oxyacids. Example sulfate (SO
4-2) comes from sulfuric acid which is H
2SO
4. The difference between the sulfate ion and sulfuric acid is the 2 hydrogen atoms.
[Edit] I'm trying to lead you in the right direction instead of just citing the 4
th rule of
forum rules, which is what I usually do when someone hasn't shown their attempt.