The atomic number of an element is defined by the number of protons in its nucleus.

Here's why this is so important:

* Protons determine the element: Every atom of a specific element will always have the same number of protons. For example, all carbon atoms have 6 protons, all oxygen atoms have 8 protons, and so on.

* Neutral atoms have equal protons and electrons: The number of protons in an atom is equal to the number of electrons, giving the atom a neutral charge. This balance is crucial for how atoms interact with each other.

* The atomic number defines the element's place on the periodic table: The periodic table is arranged in order of increasing atomic number, which reflects the fundamental nature of each element.

So, if you know the atomic number of an element, you know its identity, its number of protons, and its number of electrons in its neutral state. This is a core concept in chemistry!