# A carbon atom has a mass number of 12. How many protons and how many neutrons does it have?

The atom has $6$ $\text{protons}$, and $6$ $\text{neutrons}$.
You specified a carbon atom. By definition, the $C$ atom has 6 protons, 6 positively charged nuclear particles, in its nucleus. You further specified the ""^12C isotope. There must be 6 neutrons, 6 neutrally charged, massive particles in its nucleus.
If it's a carbon atom, the number of protons cannot change (why not?), but the number of neutrons in the nucleus can vary. ""^13C, and ""^14C isotopes are known. How many neutrons does each nuclide possess?