# How is the atomic number different from atomic mass?

$\text{The atomic number, Z,}$ is the number of nuclear protons....
And $Z$ gives the identity of the element: $\text{Z=1, hydrogen}$; $\text{Z=2, helium}$; $\text{Z=3, lithium}$;……….$\text{Z=26, iron}$....
To a first approximation, the mass number is the atomic number PLUS the number of neutrons, massive nuclear particles, that are electrically neutral. An element of given $Z$ can have an envelope of isotopes due to the fact that its nucleus may support different numbers of neutrons....