Most gases exist as molecules, for example, fluorine is F2. Explain why the elements in Group 0 exist as single atoms.

They are all stable and have eight valence electrons

Group 0 contains non-metal. The elements in group 0 are called the noble gases
They exist as single atoms, because they are all chemically unreactive.

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.