If two are atoms are isotopes of the same elements the atoms must have

The atoms must have the same number of protons.

Isotopes mean that there are atoms with the same number of protons, but a different number of neutrons.

For example, carbon 12 and carbon 14 are isotopes. Both have 6 protons, but carbon 12 has 6 neutrons, whereas carbon 14 has 8.

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 .