Atoms of the same element with different numbers of neutrons

• A. Mass number
• B. Atomic number
• C. Molar mass
• D. Isotopes

Answer: (D)

Isotopes are atoms of the same element that have different numbers of neutrons. The defining feature of an element is the number of protons (the atomic number), and that stays the same for all variants of that element. But neutrons can vary, so the total mass changes from one nucleus to another. This is why you can have atoms like carbon-12 and carbon-14: both have the same number of protons, but different numbers of neutrons, giving different mass numbers. The mass number is simply protons plus neutrons for a particular nucleus, and it varies among isotopes; the atomic number (protons) stays constant. Molar mass is a bulk property used in calculations and does not describe the identity or variant of the nucleus.