Enthalpy of formation or lattice energy for halides of alkali metals

Question

Why there is an increase in order of m.p.& b.p. in

NaF>NaCl>NaBr>NaI

Answer 1

They are all ionic compounds, and have a positive and negative attraction (coulombic force). However, all of the halogens are NOT the same SIZE.  And it is here that SIZE MATTERS.  F is the smallest of the halogens, and thus the packing of the lattice structure is "tighter", and so it takes more energy to break the lattice.  This explains why NaF has a higher melting point than, say NaCl, etc.