Why are all the gases except the ones belonging to Group VIII always in a molecular form?

Answer 1

Because they're all pretty reactive in their monoatomic state. They have incomplete outer electron shells, so they are always looking for some way to fill it up. An individual nitrogen atom (for example) will move around as a gas with fairly high kinetic energy, and it's very likely that it will run into another nitrogen atom in a similar condition. The diatomic gas molecule is at a much lower energy state, so they tend to stay together. The noble gasses, however, already have a full outer electron shell (eight electrons) so they don't have a need to react with much of anything. Don't get me started on "odd oxygen", that is either O or O3 (ozone). I'll be here all day. It exists, but only because something (like an ultraviolet photon) has knocked the O2 apart and the lonely oxygen atom hasn't yet found a long term partner.

Answer 2

Most gases form molecules because they have relatively low atomic masses and high kinetic energies, allowing them to exist as discrete units. The exception is the Group VIII noble gases (helium, neon, argon, krypton, xenon, radon), which exist as single atoms because they have full valence electron shells and are stable in their monatomic form.