The hydride ion

The enthalpy change ΔE_AH(298 K) associated with the attachment of an electron to an H atom (reaction 1.1) is ≈73 kJ mol^_1.

                                            (1.1)

All alkali metal hydrides crystallize with the NaCl lattice. From diffraction data and the ionic radii of the metal ions (Appendix 6) the radius of H_ can be estimated using equation 1.2; it varies from 130pm (in LiH) to 154pm (in CsH) and can be considered similar to that of F^- (133 pm).

             (1.2)

The large increase in size on going from the H atom (r_cov = 37 pm) to the H^- ion arises from interelectronic repulsion when a second electron enters the 1s atomic orbital. The smaller rH^- in LiH may suggest some degree of covalent bonding, but calculated and experimental values of lattice energies  for each of the group 1 metal hydrides are in good agreement,suggesting that an electrostatic model is appropriate for each compound. Hydrides of the s-block metals (excluding Be) can be made by heating the metal with H₂.

  (1.3)

When we compare ΔrH for reaction 1.3 with those for the formations of F^_ and Cl^- from F₂ and Cl₂ (^_249 and ^_228 kJ mol^-1, respectively), we understand why, since H^_ is about the same size as F^-, ionic hydrides are relatively unstable species with respect to dissociation into their constituent elements. Salt-like hydrides of metals in high oxidation states are most unlikely to exist.