Rubidium iodide has a lattice energy of -617 kJ>mol, while potassium bromide has a lattice energy of -671 kJ>mol. Why is the lattice energy of potassium bromide more exothermic than the lattice energy of rubidium iodide?

Video transcript

Hey everyone in this example, we need to explain why calcium bromide has less endo thermic lattice energy than magnesium chloride. So we should recall our periodic trend on our periodic tables for the atomic radius which is going to be increasing as we go from the bottom right of our periodic table. And so directly comparing our two molecules, we're going to compare the calcium versus magnesium and then we're going to compare our brahman to the chlorine. So what we should say for calcium versus magnesium is that according to the atomic radius, Calcium and magnesium are in the same group number. However, calcium is in period four whereas magnesium is in period three of our periodic tables. And we should recall that the period numbers are the rows going across our periodic table. And so because calcium is in period four we would say therefore it has the larger atomic radius in comparison to magnesium. And we are comparing these atoms to one another as ions. So we should recall that because calcium and magnesium are both in group two A they would form two plus cat ions, whereas bromine and chlorine are located Re recall in group seven A Where they would form therefore - and ions now comparing our bromine and chlorine and ions we want to go ahead and recognize that bro mean is located on period four, Just like calcium was of our periodic table. Whereas chlorine is located in period three. And so because it is in period four, it therefore also has the larger atomic radius based on our periodic trend for atomic radius. And so because calcium and bromine are in a molecule together in calcium bromide and they both have large atomic radius is they're not going to be able to get close to one another compared to our magnesium and chloride, which have smaller atomic radius is as they're bonded to each other in the molecule here. And so to reiterate that due to their large atomic radius is we would say that calcium and bromine are not as closely bonded compared to the smaller atomic radius is of magnesium and chlorine. And so therefore we would say that calcium and bromide ions do not release as much energy. And so that would overall make our lattice energy of the smaller atoms here in the magnesium ion and the chlorine ion within magnesium chloride have a greater value. So we would expand on our answer and say that our magnesium and chloride and ions where the magnesium Catalan and the chloride an ion are able to release more energy and therefore are more X. A. Thermic contributing to their higher value in their lattice energy. And so overall we would go ahead and confirm that the appropriate choice is going to be a because this is explained by our radius of our ions here in our molecules. So to complete this example A. Is the final choice. If you have any questions, please leave them down below. And I will see everyone in the next practice video