Question

Why does oxygen exit as a diatom molecule while the other elements in the Group 16 exist as polyatomic molecules? Calculate the number of unpaired electrons in O2+ and O2`2- ions by drawing molecular orbital diagram

Answer 1

Oxygen has 2s 2p orbitals only with a valency of -2.  So it forms O2 molecule with each atom contributing 2 electrons for sharing.

Other atoms in the group have d and f orbitals which also participate in bonding. Further they are bigger atoms. So electrons are not so tightly bonded to the nucleus.   Each atom can expose itself to a larger number of other atoms around it.  So they form polyatomic molecules.

O2 + :   O2 molecule with one positive charge... :  one unpaired electron.
          or  O^2- ?

O2^2- :  two unpaired electrons.