Question

Which of the following hydride is covalent and polymeric?

• A. $Ca{H}_2$
• B. $Be{H}_2$
• C. NaH
• D. $Ba{H}_2$

Answer 1

Answer: (B)

$Be{H}_2$

Due to small size and high Ionization energy, $Be$ always form covalent bonds, but $Be{H}_2$ is an unstable molecule due to incomplete octet so it forms polymer ($Be{H}_2$)n
Each beryllium atom is bonded to two hydrogen atoms and each hydrogen atom to two beryllium atoms. Now, beryllium has only two valence electrons and hydrogen only one, it is apparent that there are not sufficient electrons to form so many electron-pair bonds.

The monomeric $Be{H}_2$, if formed with normal bonds, would have only four electrons in the valence shell of the beryllium atom and would be electron-deficient and unstable. It polymerizes to remedy the electron deficiency.

The bonds formed cannot be explained by the classical theories of bonding. They are banana-shaped molecular orbitals holding three atoms $Be----H----Be$ together and are called $3$ centred $2-$ electron ($3c2e$) bonds. It is an example of a cluster compound.

Hence option B is correct.