Carbon has an atomic number of 6. How many protons and neutrons are in a nucleus of carbon-13?
To be carbon the atom must have 6 protons.
Each proton has a mass of 1 u for a total of 6 u.
The nucleus of
Subtracting the 6 u from the mass of the protons leaves 7 u:
Each neutron has a mass of 1 u. The 7 u left are accounted for by 7 neutrons.