Which of the following best describes the difference between mass number and atomic mass?

• A. Mass number is the number of electrons in an atom, while atomic mass is the sum of protons and neutrons.
• B. Mass number and atomic mass are identical and can be used interchangeably.
• C. Mass number is the total number of protons and neutrons in an atom, while atomic mass is the weighted average mass of all the naturally occurring isotopes of an element.
• D. Mass number is the weighted average mass of all isotopes, while atomic mass is the number of protons in an atom.

Answer: (C)

Verified step by step guidance

1. Understand that the mass number of an atom is defined as the total count of protons and neutrons in its nucleus. It is always a whole number because it is a simple count of these particles.
2. Recognize that atomic mass (also called atomic weight) is not a whole number but a weighted average of the masses of all naturally occurring isotopes of an element, taking into account their relative abundances.
3. Note that the mass number is specific to a single isotope of an element, while atomic mass reflects the average mass of all isotopes found in nature for that element.
4. Recall that electrons have negligible mass compared to protons and neutrons, so they do not significantly affect either the mass number or atomic mass.
5. Conclude that the correct distinction is: mass number is the total number of protons and neutrons in an atom, whereas atomic mass is the weighted average mass of all naturally occurring isotopes of that element.