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

Beryllium carbonate is unstable and is kept in CO₂ atmosphere to avoid decomposition. Beryllium is also the least electropositive element out of the group due to the atomic size. BeCO₃ decomposes to form BeO and CO₂ as carbonates of all alkaline earth metals decompose to give metal oxide and carbon dioxide. This reaction is reversible, hence to shift the equilibrium to the right, BeCO₃ is placed in CO₂ atmosphere.