How are diatomic molecules formed?

[LogicalAcid]

Why do some atoms bind with other atoms of their own elements, such as O2 and N2, and why do others not? What property of these atoms enable them to do this?

 

[espen180]

Are you asking about covalent bonds exclusively? The short answer is that "a full octet is more stable, so they share their electrons", but this does not cover the atoms that no not form covalent dimers, like Be. The long answer involves what are called binding and antibinding molecular orbitals. I advice you to consult any first year university chemistry textbook for an introduction to this topic.

 

[alxm]

this does not cover the atoms that no not form covalent dimers, like Be.

Ah, but Beryllium http://www.sciencemag.org/content/324/5934/1548.abstract" 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.

It's a noteworthy failure of all the simple bonding theories (MO, VB, Lewis).

 

[espen180]

Wow, thanks for that interesting read!

Judging by the references provided, all the Earth alkali metals actually form dimers! My first "instinct" is that the error of the standard bonding models arise because they don't take into account the effect of both nuclei on the electron wavefunctions. Is that the case?

 

[DrDu]

No, the nuclei aren't important. The binding in Be_2 etc. is so complicated due to strong correlation of the electrons.

 

[LogicalAcid]

No, the nuclei aren't important. The binding in Be_2 etc. is so complicated due to strong correlation of the electrons.

I still don't see how they form. I mean I know Alkaline metals are reactive, but Nitrogen and Oxygen due this. Why don't other elements? And both ionic and covalent bonds.

 

[DrDu]

I don't see the basis for your question. In gas phase, homonuclear diatomic molecules of practically all elements have been observed. With the exception of the bonding between noble gasses, these bonds are considerably stronger than van der Waals bonds, i.e. they are really covalent bonds. The classic reference is G. Herzberg, Molecular Spectra and Molecular Structure: Spectra of Diatomic Molecules

 

[Borek]

If I understand correctly what is LogicalAcid current level of education, diatomics other than those most basic (H₂, N₂, O₂ and halogens) are too esoteric to be not confusing.

It is all in energy - those commonly observed have much lover energy than atomic gases, so they dominate in conditions that we assume standard (around 1 atm and around room temperature). Those exotic ones have lower energy too, but the difference is very small, so they can survive only in gas phase and at low pressures, as in the presence of other elements they have many other ways of getting to the lowest energy state.

 

[DrDu]

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.

 

[LogicalAcid]

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.

Basically, why do some elements form diatomic molecules and others not?

 

[DrDu]

Tell me an element besides the noble gasses which does not form a diatomic molecule.

 

[LogicalAcid]

Tell me an element besides the noble gasses which does not form a diatomic molecule.

I believe Carbon does not.

 

[DrDu]

It does.