Answer
$L_{fusion}=9.8\times10^2 J/mole$
Work Step by Step
Using the hint in the problem, we see in the diagram of the simple cubic lattice that each atom has six nearest neighbors. Each bond is shared by two atoms, so the average number of bonds per atom is three (for a sufficiently large sample where the vast majority of atoms are found in the bulk, and not on the surface).
Calculate the heat of fusion from the energy required to break each bond.
$$L_{fusion}=(\frac{bonds}{atom})(\frac{atoms}{mole})(\frac{energy}{bond})$$
$$L_{fusion}=(3)(6.02\times10^{23}atoms/mole)(3.4\times10^{-3}eV)(1.60\times10^{-19}J/eV)$$
$$L_{fusion}=9.8\times10^2 J/mole$$
