Which of the following correctly describes how to determine the mass number of an atom or ion?

• A. Add the number of protons and electrons.
• B. Subtract the number of electrons from the number of neutrons.
• C. Add the number of protons and neutrons in the nucleus.
• D. Multiply the number of protons by the number of neutrons.

Answer: (C)

Verified step by step guidance

1. Understand that the mass number of an atom or ion is defined as the total number of protons and neutrons in its nucleus.
2. Recall that electrons have negligible mass compared to protons and neutrons, so they do not contribute significantly to the mass number.
3. Identify that the number of protons is the atomic number, which defines the element, and the number of neutrons can vary, leading to different isotopes.
4. To find the mass number, use the formula: \(\text{Mass Number} = \text{Number of Protons} + \text{Number of Neutrons}\).
5. Recognize that subtracting or multiplying protons and neutrons or including electrons in the calculation does not correctly give the mass number.