Which elements are not balanced in this equation MgO + 2 LiCl = MgCl2 + Li2O

A. Li

B. all elements are balanced

C. Cl

D. Mg

B All elements are balanced

This question is about stoichiometry, the ratio of compounds in a reaction. The most important thing that comes out of physics relating to this is the principle of Conservation of Energy, that is energy cannot be created or destroyed, only transferred from one form to another.

This can mean a lot of different things depending on the context, usually to explain why perpetual motion is inpossible - energy transfers are never 100% efficient and as energy cannot be expressly created, the useful energy in an isolated system must dwindle (if you are interested that is called entropy).

But in a chemistry context, for stable atoms such as these where matter remains matter and energy remains energy, it leads to the principle of Conservation of Mass. That is to say there are as many atoms of each type to start with as there are to finish with.

This is why balancing is so important. You wouldn't react two oxygen atoms together (hypothetically) to form two O2 molecules because that would start with 2 atoms and end with 4. Similarly you wouldn't (excluding nuclear fusion reactions) react two magnesium atoms together to produce a Cl2 molecule because the mass has changed (an Mg has a different mass to a Cl).

Taking note of the number of each type of atom on each side of the equation, we find there is the same number always. This shows the equation is balanced. Conservation of Mass, and by extension Conservation of Energy, is satisfied so the answer is B.