The mass number of an atom represents the total number of protons and neutrons in its nucleus. The atomic number represents the number of protons in the nucleus. To determine the number of neutrons in the nucleus, you can subtract the atomic number from the mass number.
In this case, the mass number is 13 and the atomic number is 6. Therefore, the number of neutrons can be calculated as follows:
Number of neutrons = Mass number - Atomic number = 13 - 6 = 7
So, there are 7 neutrons in the nucleus of this atom.