Ah, but Beryllium http://www.sciencemag.org/content/324/5934/1548.abstract" [Broken] form a covalent dimer! Or perhaps I should say 'covalent-ish'; there really isn't a word for that peculiar bond, although it's certainly stronger than can be accounted for by 'proper' dispersion forces (which is what most theory would predict to be the only bonding force). You could say the bond is too weak to be covalent and too short to be dispersion.this does not cover the atoms that no not form covalent dimers, like Be.
I still don't see how they form. I mean I know Alkaline metals are reactive, but Nitrogen and Oxygen due this. Why dont other elements? And both ionic and covalent bonds.No, the nuclei aren't important. The binding in Be_2 etc. is so complicated due to strong correlation of the electrons.
Basically, why do some elements form diatomic molecules and others not?You are probably right Borek, nevertheless then I do not quite understand what the question is. Maybe that not for all elements the diatomic molecules are the most stable ones in comparison with other struktures? I think LocigalAcid could precise that point.