Question

The increasing order of basic character of the oxides $MgO,\text{ }SrO,\text{ }{{K}_{2}}O\text{ and C}{{\text{s}}_{2}}O$ is:
[A] $MgO$ < $SrO$ < ${{K}_{2}}O$ < $\text{C}{{\text{s}}_{2}}O$
[B] $SrO$ < $MgO$ < $\text{C}{{\text{s}}_{2}}O$ < ${{K}_{2}}O$
[C] $C{{s}_{2}}O$ < ${{K}_{2}}O$ < $SrO$ < $MgO$
[D] ${{K}_{2}}O$ < $C{{s}_{2}}O$ < $SrO$ < $MgO$

Answer 1

HINT: Smaller atomic radii will lead to increase in covalency in the metal-oxygen bond. As a result, it will not want to donate the oxide ion thus basic character will decrease. Smaller cations will have higher covalency and thus are less basic.

Complete step by step solution:
-Before answering this question, let us try to understand the meaning of basic character.
We know that bases are electron pair donor species. The character of a species to donate its electron pair is the basic character of that species.
-In the question, the oxides of group 1 and group 2 elements are given to us. The group 1 oxides are ${{K}_{2}}O\text{ and C}{{\text{s}}_{2}}O$ and the oxides of group 2 are MgO and SrO. Now let us discuss the basic character of oxides of these two groups separately.
- Firstly, we will discuss the group 2 oxides. Basic character is basically the ability of the central metal ion to release the oxide ion. As we know down, the group size of the atomic radii increases. Among magnesium and strontium, magnesium has a smaller atomic radius than that of strontium. The smaller magnesium ion has a higher polarising power than strontium. With increase in polarising power of the cation, the covalency in the metal-oxygen bond increases. As a result of the increased covalency, it is harder for the magnesium ion to release the oxide ion. Thus, the basic character decreases and the basicity of strontium oxide is higher than magnesium oxide. Similarly, among ${{K}_{2}}O\text{ and C}{{\text{s}}_{2}}O$, polarisability of potassium is higher than cesium. Due to increased covalency in the metal-oxygen bond, it is harder for the potassium cation to release the oxide anion. Thus, the basicity of cesium oxide is higher than potassium oxide. Now, among the two groups, oxides of alkali metals are more basic than alkaline earth metals i.e. group 1 elements are more basic than group 2 elements.


Therefore, the correct answer is option [A] .

Note: There are various theories that give us the idea about base (and acid) depending upon different factors. For example, according to Arrhenius theory, hydroxide ion donors in aqueous solution are bases. According to the solvent system theory, a solute that furnishes the anion characteristic of the solvent either by direct dissociation or by reaction with solvent is a base.