# What is an atomic number?

Sep 13, 2017

By definition, the number of protons in the nucleus.

This is irrespective of the charge of the atom as well. For a given atom, we denote its isotope as:

""_(Z)^(A) X

where:

• $A$ is the mass number (sum of protons and neutrons in nucleus).
• $Z$ is the atomic number (number of protons in nucleus).
• $X$ is the chemical symbol.

So if I gave you oxygen-17,

""_(8)^(17) O,

it has $Z = 8$, and thus has $8$ protons. Does it matter if I gave you ""_(8)^(16) O, oxygen-16? Similarly, if I gave you

""_(8)^(17)O^(2-),

how many electrons does it have? (Not $8$, but . . . ?)