Question

The radius of ${\mathrm{Na}}^{+}$ is $95\mathrm{pm}$ and that of ${\mathrm{Cl}}^{-}$ ion is $181\mathrm{pm}$. Hence, what will be the coordination number of ${\mathrm{Na}}^{+}$ ?

Answer 1

Limiting Radius:

• The radius ratio is the ratio of the radius of the cat ions to that of the anions.
• Generally speaking, cations are smaller than anions.
• Anions surround the cations, which are in contact with one another.

Therefore, the limiting radius ratios provide information on:

• Ionic crystal stability
• Number of coordination
• Void geometry

Coordination number:

• The total amount of carbon, ions, or molecules linked to a certain atom is indicated by the atom's coordination number in a given molecule or crystal.
• Another name for an atom's coordination number is "ligancy."

Finding coordination number:

• Finding the limiting radius:
• $$\begin{gathered} {\mathrm{r}}^{+}=\mathrm{radius}\mathrm{of}\mathrm{cation} \\ {\mathrm{r}}^{-}=\mathrm{radius}\mathrm{of}\mathrm{anion} \end{gathered}$$

$\frac{{\mathrm{r}}^{+}}{{\mathrm{r}}^{-}}=\frac{95}{181}=0.524$

• Now after analyzing the limiting radius of the compound we find out that since the coordination number lies between $0.414-0.732$ is $6$.

Hence the coordination number of ${\mathrm{Na}}^{+}$ is $6$.