Answer:
Element X's atomic number is 30.
Explanation:
So, a nucleon number is the same as a mass number. It is the total number of neutrons and protons in the nucleus. If element X has 12 neutrons, then all we have to do is subtract 12 from 42 and we get our number of protons: 30.
Atomic number is just the number of protons in the nucleus. So, 30 is our answer.
