Which of the following best explains how one isotope of an element differs from another?

• A. They belong to different elements.
• B. They have different numbers of neutrons in the nucleus.
• C. They have different numbers of protons in the nucleus.
• D. They have different numbers of electrons in the outer shell.

Answer: (B)

Verified step by step guidance

1. Recall that isotopes are atoms of the same element, meaning they have the same number of protons in their nucleus.
2. Understand that the defining difference between isotopes lies in the number of neutrons they contain in their nucleus.
3. Recognize that the number of protons determines the element itself, so isotopes cannot differ in proton number without being different elements.
4. Note that electrons can vary in number due to ionization, but this does not define isotopes; isotopes focus on nuclear composition.
5. Therefore, the best explanation is that isotopes differ because they have different numbers of neutrons in the nucleus.