Question

Give reasons for the following:
A. `BeO` is used as a refractory material.
B. Beryllium halides are polymeric.
C. `Be(OH)_(2)` dissolves in `NaOH`, but `Ca(OH)_(2)` does not
D. On hydrolysis at room temperature, `Mg_(3)N_(2)` gives ammonia, whereas `MgCl_(2)` gives `HCl`.

Answer 1

`BeO` has high lattice enthalpy due to smaller ionic size and greater charge on `Be^(2+)` and `O^(2-)` ions. Hence, `beO` has high melting point and is used as a refractory material.
(b). Beryllium halides are electron-deficient compounds since they only four electrons in the valence shell. to complete their octet, they undergo polymerisation.
(C ). `Be(OH)_(2)` being amphoteric dissolves in `NaOH` forming sodium beryllate.
`Be(OH)_(2)+2NaOHrarrNa_(2)BeO_(2)+2H_(2)O`
Whereas `Ca(OH)_(2)` being basic, does not dissolve in `NaOH`.
(d). `Mg_(3)N_(2)` is a salt of strong base `[Mg(OH)_(2)]` and a weak acid `(NH_(3))` and hence on hydrolysis gives `NH_(3)`.
`Mg_(3)N_(2)+6H_(2)Orarr3Mg(OH)_(2)+2NH_(3)`
`MgCl_(2)` is a salt of strong base, `Mg(OH)_(2)`, and strong acid, `HCl`, and hence does not hydrolyse.