Already I know that hydrogen, all the halogens, nitrogen and oxygen forms diatomic molecules. But I am confused about $\ce{Na}$? So I would like to know about that.

  • $\begingroup$ I haven't seen any cases approving $\ce{Na2}$'s existence, but $\ce{Li2}$ exists for sure. en.wikipedia.org/wiki/Dilithium $\endgroup$ – It's Over Jun 10 '15 at 8:16
  • $\begingroup$ The molecule $\ce{Na2}$ is well known in the gas phase (and $\ce{ Li2, K2}$), and has been studied since approx 1929! The ground state has a (long) bond length of $0.3078$ nm, vibrational frequency of $159 \pu{cm^{-1}}$ and rotational constant $0.1547 \pu{cm^{-1}}$. At least four excited state are known. $\endgroup$ – porphyrin Feb 9 '17 at 16:14

According to molecular orbital theory, disodium should be stable in the gas phase, with a bond order of one. The molecular orbital diagram is the same for all the alkali metals since they all have one valence electron in an $s$ orbital.

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(image source)

NIST chemistry webbook has a small page on disodium and quotes an enthalpy change of formation of $142.07~\mathrm{kJ~mol^{-1}}$ which is moderately endothermic.

This page has a brief discussion of alkali metal dimers and claims that they are all stable in the gas phase, in agreement with the prediction from MO theory.

  • $\begingroup$ And are there any elements behave like this except what I have mentioned above and Na? $\endgroup$ – On the way to success Jun 10 '15 at 15:17

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