Which of the carbonates given below is unstable in air and is kept in CO2 atmosphere to avoid decomposition?

1. ${\mathrm{BeCO}}_{3}$

2. ${\mathrm{MgCO}}_{3}$

3. ${\mathrm{CaCO}}_{3}$

4. ${\mathrm{BaCO}}_{3}$

Hint: BeO is more stable than BeCO3

${\mathrm{BeCO}}_{3}$ is unstable to the extent that it is stable only in an atmosphere of CO2${\mathrm{BeCO}}_{3}$ shows reversible reaction because the stability of oxide formed is more than carbonates.

${\mathrm{BeCO}}_{3}⇌\mathrm{BeO}+{\mathrm{CO}}_{2}$
${\mathrm{BeCO}}_{3}$ is unstable due to the strong polarising effect of small Be2+ ion on the large polarizable carbonation. Moreover, extra
stability of the oxide is achieved through lattice energy by packing a small cation with a small oxide ion.