Here's why this is important:

* Defines the element: The number of protons is what determines the element. Every atom of a given element has the same number of protons. For example, all carbon atoms have 6 protons, all oxygen atoms have 8 protons, and so on.

* Determines chemical behavior: The number of protons dictates the number of electrons an atom has (in a neutral atom), which in turn determines how the atom will interact with other atoms to form chemical bonds.

* Indicates the number of electrons: In a neutral atom, the number of protons (atomic number) is equal to the number of electrons.

Let me know if you'd like more details about how the atomic number relates to other properties of an atom!