Why does `NO_(2)` dimerise ?

Explanation

(a) `NO_2` contains odd number of valence electrons.

It behaves as an odd electron molecule hence it dimerises. On dimerisation, it is converted to stable `N_2 O_4` molecule with even number of electrons.
(b) `PH_3` reacts with acids like `HI` to form `PH_4 I` which shows that it is basic in nature.
`PH_3 + H I rarr PH_4 I`
Due to lone pair of electrons on `P` in `PH_3`, it acts as a Lewis base in the above reaction.