In each of the symbols above, what does the bottom number tell us?

mass

Explanation:

The total mass is located at the bottom, right underneath the symbol of the element. The mass is the sum of both the protons and neutrons for both are equivalent in mass whereas the electrons have almost no mass at all.

the reaction mixture contains some ammonia, plus a lot of unreacted nitrogen and hydrogen. the mixture is cooled and compressed, causing the ammonia gas to condense into a liquid. the liquefied ammonia is separated and removed. the unreacted nitrogen and hydrogen are then recycled back into the reactor .

elements make up everything, and they combine to make compounds.

the answer is c