The charge/size ratio of a cation determines its polarising power....

The charge/size ratio of a cation determines its polarising power. Which one of the following sequences represents the increasing order of the polarising power of the cationic species,

\mathrm{K}^{+}, \mathrm{Ca}^{2+}, \mathrm{Mg}^{2+}, \mathrm{Be}^{2+}

Options

A $\mathrm{K}^{+} \lt \mathrm{Ca}^{2+} \lt \mathrm{Mg}^{2+} \lt \mathrm{Be}^{2+}$
B $\mathrm{Be}^{2+} \lt \mathrm{K}^{2+} \lt \mathrm{Ca}^{2+} \lt \mathrm{Mg}^{2+}$
C $\mathrm{Mg}^{2+} \lt \mathrm{Be}^{2+} \lt \mathrm{K}^{+} \lt \mathrm{Ca}^{2+}$
D $\mathrm{Ca}^{2+} \lt \mathrm{Mg}^{2+} \lt \mathrm{Be}^{+} \lt \mathrm{K}^{+}$

Correct answer

A. $\mathrm{K}^{+} \lt \mathrm{Ca}^{2+} \lt \mathrm{Mg}^{2+} \lt \mathrm{Be}^{2+}$

Step-by-step solution

High charge and small size of the cations increases polarisation. As the size of the given cations decreases as

\mathrm{K}^{+}\gt\mathrm{Ca}^{2+}\gt\mathrm{Mg}^{2+}\gt\mathrm{Be}^{2+}

Thus, polarising power increases as

\mathrm{K}^{+} \lt \mathrm{Ca}^{2+} \lt \mathrm{Mg}^{2+} \lt \mathrm{Be}^{2+}

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