# What is the atomic symbol for isotopes with the following characteristics: 15 protons and 16 neutrons?

$\text{Well, it is an isotope of phosphorus.............}$
If there are 15 protons, 15 nuclear particles of unit positive charge, then $Z = 15$. Now $Z \equiv \text{the atomic number}$, and you look at your copy of the Periodic Table, and you find that for $Z = 15$, the element phosphorus is specified.
But we are not finished. Along with the 15 defining protons, there are also 16 neutrally charged, massive nuclear particles, 16 neutrons, and the protons and neutrons together determine the atomic mass. The isotope is thus ""^31P, which is almost 100% abundant, and an important nucleus for $\text{NMR spectroscopy.}$