Question

What is an atomic number?

Answer 1

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 . . . ?)