Which of the following contains both covalent and ionic bonds?

Amongst the following,the total number of species $NOT$ having $8$ electrons around the central atom in its outermost shell is $NH_3$,$AlCl_3$,$BeCl_2$,$CCl_4$,$PCl_5$:

In the formation of a molecule by atoms,which forces operate?

The order of the average bond length of the given bonds is

Which of the following pairs does not contain isoelectronic species?

The number of $S-S$ bonds in sulphur trioxide trimer $S_3O_9$ is