Question

All alkali and alkaline earth metals form ionic hydrides except Be and Mg WHY?? Please reasons, thanks

Answer 1

Two reasons: (a) ionization energies and (b) electronegativities.

Explanation:

(a) Ionization energies

Of all the Group 1 and Group 2 metals, `"Be"` and `"Mg"` have the highest ionization energies (899.5 kJ/mol and 737.7 kJ/mol, respectively).

Thus, it is more difficult to convert them into cations.

(b) Electronegativities

Of all the Group 1 and Group 2 metals, `"Be"` and `"Mg"` have the highest electronegativities (1.57 and 1.31, respectively).

Thus, their electronegativities are the closest to that of `"H"` (2.20).

They are therefore the most likely candidates to have covalent `"M-H"` bonds.

Calculations from electronegativity differences predict a `"Be-H"` bond to have
only 9 % ionic character and an `"Mg-H"` bond to have 18 % ionic character.

The structure of `"BeH"_2`

Beryllium hydride forms polymeric chains in which the `"H"` atoms
form 3-centred 2-electron bonds to the next beryllium atom in the chain.

Here's a model of a chain segment.

(From www.chemtube3d.com)

The structure of `"MgH"_2`

`"MgH"_2` has a crystal structure in which the `"Mg"` atoms (blue) form a distorted body-centred cubic unit cell.

The `"Mg"` is almost completely ionized (`q = "+1.95e"`).

However, the `"H"` atoms do not have a corresponding charge of `"-1e"`.

Instead there is a significant amount of charge density between `"Mg"` and `"H"`, indicating a somewhat covalent `"Mg-H"` bond.