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

1. BeCO3

2. MgCO3

3. CaCO3

4. BaCO3

Hint: BeO is more stable than BeCO3
BeCO3 is unstable to the extent that it is stable only in an atmosphere of CO2BeCO3 shows reversible reaction because the stability of oxide formed is more than carbonates.
BeCO3 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.