The correct answer is A. the same number of protons but different number of neutrons.
Isotopes are atoms of the same element that have different numbers of neutrons. They have the same number of protons, which determines the element’s atomic number, but different numbers of neutrons, which determines the element’s mass number. For example, carbon-12 has 6 protons and 6 neutrons, while carbon-14 has 6 protons and 8 neutrons.
Protons and neutrons are both found in the nucleus of an atom. The number of protons determines the element’s chemical properties, while the number of neutrons affects the atom’s stability. Isotopes of the same element have the same chemical properties, but they may have different physical properties. For example, carbon-12 is a stable isotope, while carbon-14 is radioactive.
The other options are incorrect. Option B is incorrect because isotopes have the same number of protons, not neutrons. Option C is incorrect because isotopes have the same number of protons, not electrons. Option D is incorrect because isotopes have the same number of protons and neutrons, not nucleons.