The nucleus — Explain

What is meant by an isotope, and can one element have more than one isotope?

Isotopes are atoms of the same element (same proton/atomic number, so the same number of protons) that have different numbers of neutrons, and therefore different nucleon (mass) numbers. Yes - an element may have two or more isotopes, e.g. carbon-12 and carbon-14 both have 6 protons but have 6 and 8 neutrons respectively.