Question

1. what does the number next to isotopes signify (ie: carbon 12 vs. carbon 13)?

Explanation:

Isotopes are atoms of the same element with different numbers of neutrons. The number next to an isotope (e.g., 12 in Carbon - 12) is the mass number, which is the sum of protons and neutrons in the nucleus of the atom.

Answer:

The mass number, which is the sum of protons and neutrons in the nucleus of the atom.