What does the number next to an isotope (for example, carbon-14) signify?

• A. The number of electrons in the atom
• B. The total number of protons and neutrons in the nucleus
• C. The number of protons in the nucleus
• D. The atomic number of the element

Answer: (B)

Verified step by step guidance

1. Understand that isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.
2. Recall that the number next to an isotope's name (like the 14 in carbon-14) is called the mass number.
3. The mass number represents the total number of protons and neutrons in the nucleus of the atom.
4. Since the number of protons defines the element, the difference in isotopes comes from varying numbers of neutrons, which changes the mass number.
5. Therefore, the number next to an isotope indicates the sum of protons and neutrons, not just protons, electrons, or the atomic number.