Isotope

Not to be confused with Isopod; Isomer; Allotrope; Isobar; or probably some other shit, too.

An isotope is a distinct nuclear species of the same chemical element. They have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), but different mass numbers due to different numbers of neutrons in their nuclei. While all isotopes of a given element have similar chemical properties, they have different atomic masses and physical properties because of course chemistry had to make this more complicated.

FUCK CHEMISTRY

Unless you're in Springfield. Or Albuquerque. Or Waltersburg.