I was wondering, since 4 out of the five Halogens are diatomic, why isn't Astatine? I mean they are supposed to have similar characteristics right? Also, what causes an element to be diatomic/polyatomic in the first place? I've been googling for answers but I can't seem to find any, so any help here guys?
The short answer is that being monatomic/diatomic/polyatomic has to do with the valence electrons shell. The valence shell is attracted by the nucleus, but this attraction becomes weakened the further out you go, because there's more electrons between the outer shell and the nucleus, this is why, in the case of the Group 1 metals, the reactivity increases as you move down the group - because it becomes increasingly easy for them to loose that electron. In the case of the Halogens, it also explains why the reactivity decreases as you move down the group, because it becomes harder for them to get that last electron from other things. In the case of Astatine, it has (about) the same electronegativity as Gold, Iridium, Platinum, and Hydrogen. It also has about the same first ionization energy as gold, iridium, and platinum. The shapes of the orbitals, and the ability of the orbitals to intefere constructively and destructively with each other also plays a significant role, as does (I think) the fact that as you move down a group, the orbitals of different shape become increasingly closer in energy. My recollection is that it's a combination of all of the above that leads to increasing metalic properties as you move down a group - Consider Group 14 which starts with Carbon and ends with Lead. Or even Group 15 which starts with Nitrogen and ends with Bismuth.
