The atomic number is determined by the number of protons in an atom's nucleus.

Here's why:

* Protons are positively charged particles found in the nucleus of an atom.

* Electrons are negatively charged particles that orbit the nucleus.

* Neutrons are neutral particles also found in the nucleus.

The number of protons defines the element. For example, all carbon atoms have 6 protons, all oxygen atoms have 8 protons, and so on. The number of neutrons can vary, leading to isotopes of the same element, but the number of protons always stays the same.