Coordination number of ${\mathrm{Na}}^{+}$  ion is $\mathrm{NaCl}$  is 6 and that of Cesium ion in $\mathrm{CsCl}$  is 8. This is because

Size of ${\mathrm{Na}}^{+}$  is less than  $C{s}^{+}$

Attraction of ${\mathrm{Na}}^{+}$  is higher than  $C{s}^{+}$

$\mathrm{IP}\text{\hspace{0.17em}\hspace{0.17em}of\hspace{0.17em}\hspace{0.17em}}\mathrm{Cs}$  is less than  $\mathrm{Na}$

None