Here's why:

* Atomic Number: The number of protons in an atom's nucleus determines its atomic number. This number is unique to each element. For example, all carbon atoms have 6 protons, while all oxygen atoms have 8 protons.

* Element Identity: The atomic number directly dictates the element's identity. Changing the number of protons fundamentally changes the element.

While neutrons also reside in the nucleus and contribute to an atom's mass, they don't define the element. Isotopes of the same element can have varying numbers of neutrons.