Question

Help Please

Which of the following would have the greatest magnitude for lattice energy?

a.LiCl
b.MgF2
c.NaF
dMgI2
e.KBr

The answer is MgF2, but I don’t understand why MgF2 should have the greatest Lattice energy, because from the other molecules it is LiCl where Cl can hold tightly Li, Li have less electronegativity and, therefore, it should takes much energy to brake them than MgF2?

Answer 1

Fluorine is the most electronegative element, much more so than chlorine. The only other answer involving fluorine (c) can be ruled out because there are only +1 and -1 charges. Mg(+2) and F(-1) will be bound more tightly.

Answer 2

Electronegativity is not relevant in this context. The only important factors are the charge on the ions and their size, because the force holding the lattice together is electrostatic attraction. You'll recall from Coulomb's Law that the strength of electrostatic attraction is directly proportional to the charge and inversely proportional to the distance between the charges (F = q1*q2/r^2).

So first consider the charges on the ions:

a. Li+ and Cl-
b. Mg+2 and F-
c. Na+ and F-
d. Mg+2 and I-
e. K+ and Br-

On this basis, the two leading candidates are b and d, because both involve Mg+2. Twice as large a charge on the cation means twice as strong an electrostatic attraction.

Now consider the sizes of the ions. Since F is in Period 2 and I is in Period 5, the F- anion will be much smaller than the I- anion. That will allow the cation and anion to get much closer -- which means a much stronger electrostatic attraction.

So your final answer is b, MgF2.