Question

Which one of the carbonates is unstable in air and is kept in $CO_{2}$ atmosphere to avoid decomposition?

• A. $MgCO_{3}$
• B. $CaCO_{3}$
• C. $BaCO_{3}$
• D. $BeCO_{3}$

Hint:

Smallest cation ($Be^{2+}$) leads to highest polarizing power, causing the lowest thermal stability among group 2 carbonates.

Answer 1

The Correct Option is D

Step 1: Recall the stability trend.
For group 2 carbonates, thermal stability rises as we go down the group. This is because the metal ion gets bigger and pulls less on the carbonate.

Step 2: Link size to polarising power.
A small cation has high polarising power. It distorts the carbonate ion strongly and helps it break into an oxide and carbon dioxide.

Step 3: Find the smallest cation.
Among Mg, Ca, Ba and Be, the beryllium ion $Be^{2+}$ is the smallest. So $BeCO_3$ has the least thermal stability.

Step 4: See what this means.
Because $BeCO_3$ is so unstable, it tends to break down into BeO and $CO_2$ even at room temperature.

Step 5: Explain the $CO_2$ atmosphere.
By keeping it under a carbon dioxide atmosphere, the extra $CO_2$ pushes the reaction backward and stops the carbonate from decomposing.

Step 6: Pick the answer.
So the carbonate that must be stored under $CO_2$ is beryllium carbonate. \[ \boxed{BeCO_3} \]