Question

Which one of the following substances would you expect to have the largest lattice energy (absolute value)? a) CsI b) RbCl c) NaBr d) NaI e) NaF

Answer 1

e) NaF has the largest lattice energy.

The lattice energy depends on the attraction between the oppositely charged ions.

The force of attraction `F` is directly proportional to the charges (`q_1` and `q_2`) on the two ions and on the distance `r` between them.

`F = (q_1q_2)/r^2`

In all of your compounds, the charges are +1 and -1, so the differences must arise from the distances between the ions.

The smallest ions will be able to get closest to each other.

They will have the smallest distance between centres and will have the largest lattice energies.

The smallest atoms are at the top of the Periodic Table.

Of Cs⁺, Rb⁺, and Na⁺, the smallest ion is Na⁺.

Of I⁻, Cl⁻, Br⁻, and F⁻, F⁻ is the smallest anion.

NaF contains the smallest cation and the smallest anion, so NaF has the largest lattice energy.