The number of protons in the nucleus of an atom is called the atomic number.

Here's why this is important:

* Defines the element: The atomic number uniquely identifies an element. For example, all carbon atoms have 6 protons, all oxygen atoms have 8 protons, and so on.

* Determines the chemical properties: The number of protons in an atom's nucleus dictates how many electrons it has, and the arrangement of these electrons determines the element's chemical behavior.

Let me know if you want to know the atomic number of a specific element!