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To me it seems like diatomic elements like $\ce{F2}$, $\ce{Cl2}$, etc. occur more prevalently than diatomic molecules with both, like $\ce{ClF}$.

Is this the case, and if so, what causes diatomic elements to be favored?

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There's a lot of circumstantial evidence that heterodiatomics are more stable than homodiatomics. It is usually ascribed to the observation that atoms of different "electronegativity" tend to have stronger bonds than atoms of the same electronegativity.

In your particular example, consider the reaction:

$$\ce{Cl2 + F2 -> 2 ClF}$$

I don't have the tables handy, but I calculate with electronic structure theory that this reaction is exothermic by about 25 kcal/mol. That's quite a bit! (The equilibrium lies far to the right.)

This would suggest that a mixture of Chlorine and Fluorine, heated up to very high temperature and then cooled down, would tend to reform as ClF.

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  • $\begingroup$ Thanks! But why don't we find ClF more frequently then? Do they simply not have enough energy to combine? $\endgroup$ – DarkLightA May 10 '13 at 7:31
  • $\begingroup$ @DarkLightA Might be that the activation energy is very high? I can't seem to find it though to get an actual value. $\endgroup$ – Jerry May 10 '13 at 9:13
  • $\begingroup$ Alright, I guess it might be like ionic triple bonding then? Takes a lot of energy to obtain, but once in the bonding, is hard to break? $\endgroup$ – DarkLightA May 10 '13 at 9:22
  • $\begingroup$ I have not seen evidence that $\ce{Cl2}$ and $\ce{F2}$ are more prevalent in nature than $\ce{ClF}$. They may be synthesized for particular purposes more often, or they may be talked about more often--but that's a totally different matter. $\endgroup$ – Eric Brown May 10 '13 at 11:42
  • $\begingroup$ Please see my answer, which addresses the activation energy point. Heat up very hot (thereby overcoming activation barrier) and then cool down (thus forming thermodynamically-stable product). Again, I don't know if there is some reaction in nature that preferentially forms diatomic halogens--but I highly doubt there is. Heterodiatomics are usually more stable than homodiatomics. If you have some basis for assuming homodiatomic halogens are more prevalent in nature, then please cite your evidence/context (e.g. observed more in the atmosphere by some spectroscopic method) $\endgroup$ – Eric Brown May 10 '13 at 12:17

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