# What is an atomic number?

The atomic number, $Z$, is the number of protons, massive positively charged particles, contained in the element's nucleus.
$Z$ defines the identity of the element. If $Z$ $=$ $1$, the element is hydrogen; if $Z$ $=$ $2$, the element is helium; if $Z$ $=$ $19$, the element is potassium; if $Z$ $=$ $92$, the element is uranium.