# If an atom contains 11 protons and 12 neutrons, what is its atomic number?

$Z , \text{ the atomic number}$ $=$ $11.$
$Z$ is specified by the number of nuclear protons; i.e. the number of positively charged nuclear particles. $Z$ specifies the identity of the nucleus. If $Z = 11$, then the element is sodium. We would thus describe this species as ""^23Na.