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Atomic number (z) = number of protons

Atomic mass = mass of protons + mass of neutrons
Note:  relative atomic mass is the average of the atomic masses of an element's isotopes.  For example, the relative atomic mass of carbon is 12.01 amu.  The explanation is that the isotopic abundance is as follows:  ¹²C (stable)= 98.9%; ¹³C (stable)= 1.1 %; ¹⁴C (Beta, 5730 years)= trace.