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I know hydrogen forms a diatomic molecule $\ce{H2}$, where the electronic configuration of hydrogen is $\ce{1s}$.

But why doesn't lithium also form a diatomic molecule? Its electronic structure is $\ce{1s^2 2s}$, so can't two lithium atoms come together, share their outer electron and form $\ce{Li2}$? Same with $\ce{Na}$, $\ce{K}$ etc.

Am I missing something obvious?

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  • $\begingroup$ Note that this is one of the properties that is usually taught at elementary level to differentiate hydrogen from alkali metals, even though they occupy the same group. In fact, some periodic tables depict hydrogen with a "dual position" - in both group 1 and group 17, since it shares several properties of both alkali metals and halogens. I'll spare the details for a high school textbook and this quora page. $\endgroup$ – Gaurang Tandon Feb 24 '18 at 16:12
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    $\begingroup$ Strongly related: chemistry.stackexchange.com/questions/32686/… $\endgroup$ – Nilay Ghosh Feb 24 '18 at 16:23
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Diatomic molecules of alkali metals are detected in the gas phase. However, it's so happens that the bond in them is very weak and at the temperature the alkali metal vapors develop only a few percents of the metal in the vapors exists as diatomic molecules. It so happens, that metal bonding allows to achieve an overall more energetically favored state.

Outer orbitals of the alkali metal atoms are very diffuse, so their bonds are weak. Only Lithium, the smallest of the alkali metals, has practically meaningful covalent chemistry to speak of.

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