Question

All alkali and alkaline earth metals form ionic hydrides except Be and Mg. Why?

Answer 1

Ionic compounds are formed due to the electrostatic interactions between cations and anions. Ionic hydride refers to the compound in which the hydride ion $({H^ - })$ and a cation is present. These compounds are formed when there is a large electronegativity difference between the hydrogen atom and the metal atom. So, now we will see the factors that cause the Beryllium hydride and Magnesium hydride to be covalent in nature.

Complete Step By Step Answer:
We know that to form an ionic hydride compound, the electronegativity difference between the hydrogen atom and the metal atom should be high. Another factor that can be present to cause the Beryllium hydride and Magnesium hydride to be covalent in nature is ionisation energies of both Beryllium and Magnesium.
- Electronegativity difference: Now, the electronegativities of beryllium and magnesium are $1.57$ and $1.31$ , respectively which are very close to the electronegativity of hydrogen which is $2.20.$ Therefore, the hydrides of Be and Mg are not ionic in nature.
- Ionization energies: Out of all the group $1$ and group $2$ elements, Be and Mg have highest ionization energies and hence, it will be difficult to convert them into ions. So, the nature of hydrides of Be and Mg will be covalent.

Note:
We should note that covalent compounds are formed when the electronegativities of both the atoms are similar. Covalent bonds are formed when the electrons are shared between the two atoms while ionic bonds are formed when the electrons get transferred from the metal atom to the non-metal atom.