Bond lengths

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If the valence electrons in a metal are spread over a large number of bonds, each bond should be weaker and hence longer. The alkali metals exist as diatomic molecules in the vapour state, and the interatomic distances in the metal crystal are longer than in the diatomic molecule. Though the bonds in the metal are longer and weaker, there are many more of them than in the M2 molecule, so the total bonding energy is greater in the metal crystal. This can ·be seen by comparing the enthalpy of sublimation of the metal crystal with the enthalpy of dissociation of the M2 molecules.






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