Isotopes of an element differ in the number of which subatomic particle?

• A. Valence electrons (in a neutral atom)
• B. Electrons (in a neutral atom)
• C. Protons
• D. Neutrons

Answer: (D)

Verified step by step guidance

1. Recall that isotopes are atoms of the same element that have the same number of protons but differ in the number of neutrons.
2. Understand that the number of protons defines the element and does not change between isotopes.
3. Recognize that electrons can vary in ions, but in neutral atoms, the number of electrons equals the number of protons and does not define isotopes.
4. Valence electrons refer to the electrons in the outermost shell and can vary due to chemical bonding, but this variation does not define isotopes.
5. Therefore, isotopes differ specifically in the number of neutrons, which affects the atomic mass but not the chemical identity of the element.