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Ok my understanding of the electronic structure of atoms could be wrong but this is why I don't understand:Why alkali metals are so much electropositive.

Some high school professor in chemistry would say because if they lose an electron they have an electronic configuration of noble gas etc.

If alkali metals gained 1 electron then the wavefunction of the each electron in the s orbital would shield some of the charge of the nucleus for the other electron in the s orbital->the overall net charge on each electron would be decreased but because now we have 2 electrons instead of one potential energy would be decreased.

That is clearly not the case.Why?

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    $\begingroup$ Dimers of Sodium or Potassium atoms do exist in the gas phase and in the absence of air. But the proportion of dimer to the total of atomes is weak. $\endgroup$ – Maurice May 28 '20 at 12:50
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Maurice notes that alkali metal diatomic molecules are at best a very minor constituent. But alkali metals do fill their $s$ valence shells in other ways.

Most alkali metals are known to form alkalides, in which an electron is transferred from one alkali metal to another (or from barium to an alkali metal, in a more recent example). To effect this electron transfer, a complexing agent is used to stabilize the cation, such as 2.2.2 cryptand in the compound $\ce{[Na}\text{(2.2.2-crpytand)}\ce{]^+Na^-}$. A combination of low ionization energy, positive electron affinity and the stabilization offered by complexing the cation enable this outcome.

Lithium apparently fails to form such alkalides, but not because it refuses to adopt an $s^2$ valence electron configuration. Quite the opposite: it tends to form polar covalent bonds with that configuration. Organolithium compounds are a widely used example of this electron structure, and the covalent bonding component in lithium salts may contribute to their solubility in some organic solvents.

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