The atomic number of an element is determined by the number of protons in the nucleus of an atom of that element.

Here's why:

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

* The number of protons defines the element. For example, all carbon atoms have 6 protons, and all oxygen atoms have 8 protons.

* Neutrons are neutral particles also found in the nucleus. The number of neutrons can vary within an element (leading to isotopes), but the number of protons remains constant.

* Electrons are negatively charged particles that orbit the nucleus. The number of electrons in a neutral atom is equal to the number of protons.

Therefore, the atomic number is a fundamental property of an element, and it is what distinguishes one element from another.