Question

current attempt in progress
how is it possible for there to be more than one kind of atom of the same element?
the properties of an element are related to the number of protons and electrons. if the number of isotopes result. isotopes of an element are still the same element even though the composition of the atoms are different.
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Explanation:

Elements are defined by proton - number. Isotopes of an element have the same number of protons but different numbers of neutrons, resulting in different atomic mass compositions while remaining the same element.

Answer:

neutrons; atomic mass