Here's why:

* Protons define the element: The number of protons in an atom's nucleus is its atomic number. This number uniquely identifies an element. For example, all carbon atoms have 6 protons, while all oxygen atoms have 8 protons.

* Isotopes: While the number of protons defines an element, atoms of the same element can have different numbers of neutrons. These are called isotopes. For example, carbon-12 and carbon-14 are both isotopes of carbon, with 6 protons each but different numbers of neutrons.

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