The subatomic particle that determines what element an atom is 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.

* Defining Element: The atomic number essentially defines what element an atom is. For example, all carbon atoms have 6 protons, while all oxygen atoms have 8 protons.

* Isotopes: While the number of neutrons can vary within an element (creating isotopes), the number of protons always stays the same.

So, the number of protons is the key identifier of an element.