The correct answer is "different ions of an element."
Here's why:
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Different ions of an element: Ions are formed when an atom gains or loses electrons. Therefore, different ions of the same element have different numbers of electrons.
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Different isotopes of an element: Isotopes of an element have the same number of protons (and hence the same number of electrons when neutral) but different numbers of neutrons. Therefore, they have the same number of electrons.
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The nucleus of an element: The nucleus contains protons and neutrons. The number of protons (atomic number) determines the element, and the number of neutrons can vary (isotopes), but the electrons are not part of the nucleus.
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Each of the atoms of an element: All atoms of a neutral element have the same number of electrons as protons. Thus, they have the same number of electrons.
In summary, only different ions of an element have different numbers of electrons.