Question

1. how do you know if an element is an isotope?

Explanation:

Isotopes are atoms of the same element with different numbers of neutrons. So, if atoms of an element have varying mass - numbers (due to different neutron counts) while having the same atomic number (same number of protons), they are isotopes.

Answer:

If atoms of an element have the same atomic number but different mass - numbers (owing to different numbers of neutrons), they are isotopes.