Two atoms that are isotopes of one another must have a different number of which subatomic particle?

• A. Protons
• B. Electrons
• C. Quarks
• D. Neutrons

Answer: (D)

Verified step by step guidance

1. Recall that isotopes are atoms of the same element, meaning they have the same number of protons.
2. Since the number of protons defines the element, isotopes cannot differ in protons.
3. Electrons can vary in ions, but isotopes specifically refer to differences in the nucleus, so electrons are not the differing particle.
4. Quarks are fundamental particles inside protons and neutrons, but isotopes are defined by differences at the nuclear particle level, not quarks.
5. Therefore, isotopes differ in the number of neutrons, which changes the atomic mass but not the atomic number.