The atomic number of a nucleus is simply the number of protons it contains.

Here's why:

* Atomic Number: Defines the element. Every element has a unique number of protons. For example, all carbon atoms have 6 protons, making their atomic number 6.

* Nucleus: The central part of an atom, containing protons and neutrons.

* Protons: Positively charged particles within the nucleus.

Therefore, the atomic number is a fundamental property of the nucleus.