Final answer:
Elements are defined by the number of protons in the nucleus of an atom.
Step-by-step explanation:
Elements are defined by the number of protons in the nucleus of an atom. This number, called the atomic number, determines the identity of the element. For example, all hydrogen atoms have one proton, while all carbon atoms have six protons. The number of neutrons and electrons can vary among atoms of the same element, but the number of protons remains constant.
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