The subatomic particle that identifies an atom of a particular element is the proton.

Here's why:

* Atomic Number: The number of protons in an atom's nucleus is called its atomic number. This number is unique to each element.

* Element Identity: Since the atomic number defines the number of protons, it directly determines the element. For example, all carbon atoms have 6 protons, while all oxygen atoms have 8 protons.

While the number of neutrons can vary (creating isotopes of the same element), the number of protons remains constant and defines the element.