Here's why it's important:

* Defines the element: Each element has a unique atomic number. For example, all carbon atoms have 6 protons, and all oxygen atoms have 8 protons.

* Determines chemical properties: The number of protons dictates how many electrons an atom has (in a neutral atom), and this determines how the atom will interact with other atoms.

* Relates to isotopes: While the number of protons defines the element, the number of neutrons can vary, creating isotopes of the same element.

In short, the atomic number is the fundamental identifier of an element and its chemical behavior.