Lattice Energy Strongest to Weakest Bonds

Question

In an ionic compound, the size of the ions affects the internuclear distance (the distance between the centers of adjacent ions), which affects lattice energy (a measure of the attractive force holding those ions together).

Based on ion sizes, arrange these compounds by their expected lattice energy.

Note that many sources define lattice energies as negative values. Please arrange by magnitude and ignore the sign.

∣lattice energy∣=absolute value of the lattice energy

Arrange BaO, CaO, MgO, BeO, and SrO in greatest lattice energy (strongest bonds) to least lattice energy (weakest bonds)

Answer 1

Lattice Energy, as the problem states, is affected by the internuclear distance between ions. However, the problem does not tell us whether increasing the distance between ions causes an increase or a decrease in lattice energy, so we'll have to figure that out ourselves. Lattice energy is the result of electrostatic attraction between ions, and because these attractive forces decrease with increasing distance between ions, the magnitude of the lattice energy decreases with increasing distance between ions. In other words, compounds with bigger ions should have weaker lattice energy than compounds with smaller ions.

With this in mind, we just need to determine which ions are bigger and which ones are smaller, and order the compounds from smallest (which have the greatest lattice energy) to biggest (which have least lattice energy). All of these compounds have oxygen as the anion (the negatively charged ion), so the size difference will come from the cation (the positively charged ion). Looking at a periodic table, we can see all of the cations are elements from the Column II, which are known as the "alkaline earth metals". The elements further down on the periodic table have bigger nuclei and further out electron orbitals than those further up on the table, and so we would expect the elements lower down on the table to form the biggest ions. The column reads, starting from the top, Beryllium, Magnesium, Calcium, Strontium, Barium, Radium, and so the compounds should be ordered: BeO, MgO, CaO, SrO, BaO.