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

1 Answer
Jul 24, 2017

Answer:

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#

s3mn.mnimgs.com

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.

BeH2
(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.

pubs.rsc.org

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.