Assertion : An important feature of Fluorite structures is that cations being large in size occupy FCC

lattice points whereas anions occupy all the tetrahedral voids giving the formula unit $A{B}_2$(A: cation B: anion).

Reason : There are 6 cations and 12 anions per FCC unit cell of the Fluorite structure.

Assertion : In NaCl crystal each Na ion is touching 6 $C{l}^{‐}$ ions but these $C{l}^{‐}$ ions do not touch each other.

Reason : The radius ratio roe $r_{Na}/r_{Cl-}$ is greater than 0.414, required for exact fitting.

Assertion : The ratio of $Z{n}^{2+ }$and F- per unit cell of ZnS and $Ca{F}_2$ is 1:2. 

Reason : In ZnS $Z{n}^{2+}$ ions occupy alternate tetrahedral voids and in $Ca{F}_2$ F- ions occupy all the tetrahedral voids of FCC unit cell.

Assertion :  The stability of a crystal gets reflected in its melting point.

Reason :  The stability of a crystal depends upon the strength of the interparticle attractive force. The melting point of a solid depends on the strength of the attractive force acting between the constitutent particles.

Assertion :  The melting point decreases in the order.
Water > ethyl alcohol > diethylether > methane

Reason : The strength of the intermolecular forces between these molecules follow the order
Water > ethyl alcohol > diethylether > methane