# Acommon isotope of Carbon has an atomic number of and a mass number of 13. How many neutrons are in A 6 an atom of this isotope?

There are necessarily $7$ neutrons in the ""^13C isotope.
All carbon nuclei contain 6 nuclear protons; i.e 6 positively charged, massive nuclear particles. Were there a different number than 6, then we could not call it a carbon isotope, i.e for the $C$ nucleus, $Z = 6$.
Most carbon nuclei also contain 6 neutrons, 6 massive, neutrally charged nuclear particles; i.e. the ""^12C isotope. A few carbon nuclei contain 7 neutrons rather than 6 to give the ""^13C isotope; the isotopic abundance is approx. 1.1%.
Chemists can now routinely take advantage of the magnetic properties of the ""^13C isotope by means of ""^13C{""^1H} $\text{NMR spectroscopy}$. This is a bit less sensitive than ""^1H $\text{NMR spectroscopy}$, and a bit of a longer experiment, but it is a direct method of analysis, and characterization of a new organic molecule would routinely demand details of its ""^13C{""^1H} $\text{NMR spectrum}$.