Question

Which of the following alkaline earth metal carbonates is the thermally most stable?
a.) $MgC{{O}_{3}}$
b.) $CaC{{O}_{3}}$
c.) $SrC{{O}_{3}}$
d.) $BaC{{O}_{3}}$

Answer 1

Hint: The elements of Group 2 of the modern periodic table are called alkaline earth metals because their oxides are alkaline in nature and remain unaffected by the heat of earth crust.

Complete step by step solution:

Beryllium (Be), Magnesium (Mg), Calcium (Ca), Strontium (Sr), Barium (Ba) and Radium (Ra) are the elements present in the Group 2 of the modern periodic table. All the elements have two electrons in their valence shells, which gives them an oxidation state of +2.
We can see the following trends when we move down the group:
The elements become less electronegative.
The bonding pair is increasingly attracted away from the element toward the more electronegative element.
In other words, electropositivity of the metal increases as we move down the group.

Carbonates in this group undergo thermal decomposition to the metal oxide and carbon dioxide gas and the size of the cation present in the carbonate increases as we move down the group.
$XC{{O}_{3\,}}\,(s)\,\to \,XO\,(s)\,+\,C{{O}_{2}}\,(g)$
As a result, it requires more thermal heat to decompose. So, carbonates become more and more thermally stable down the group.
$MgC{{O}_{3}}\, < \,CaC{{O}_{3}}\, < SrC{{O}_{3}}\, < \,BaC{{O}_{3}}$
Therefore, the correct answer is (d).

Note: Group-2 carbonates are sparingly soluble in water due to their increase in covalent nature. Though, no clear solubility trend is observed. But alkaline earth metal carbonates are soluble in a solution of $C{{O}_{2}}$ due to formation of $HCO_{3}^{-}$.