# How does the number of nuclear particles relate to the identity of the atom, and the identity of the isotope?

The number of PROTONS, massive, positively charged nuclear particles, gives $Z$, the atomic number, and determines the identity of the element. The nucleus MAY contain various numbers of NEUTRONS, massive, neutral particles, and this gives rise to the existence of isotopes, atoms of the SAME element but with different mass. See this old answer.