If #Z=6# you specify the carbon element UNEQUIVOCALLY. The most abundant isotope is #""_6^12C#, i.e. 6 neutrons in the carbon nucleus, the next most abundant is #""_6^13C#, which allows non-sporting analysis of carbon compounds by NMR spectroscopy. #""_6^14C# has a half-life of 5700 years...........