Isotope Isotope Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in mass number. All isotopes of a given element have the same number of protons but different numbers of neutrons in each atom. The term isotope is formed from the Greek roots isos (ἴσος "equal") and topos (τόπος "place"), meaning "the same place"; thus, the meaning behind the name is that different isotopes of a single element occupy the same position on the periodic table. The number of protons within the atom's nucleus is called atomic number and is equal to the number of
Isomer the same degree but in opposite directions. Other types of isomerism exist outside this scope. In general, topological isomers called topoisomers are large molecules that wind about and form different-shaped knots or loops. Molecules with topoisomers include catenanes and DNA. Topoisomerase enzymes can knot DNA and thus change its topology. There are also isotopomers or isotopic isomers that have the same numbers of each type of isotopic substitution but in chemically different positions. In nuclear physics, nuclear isomers are excited states of atomic nuclei. Spin isomers have differing distributions of spin among their constituent atoms. Isomer An isomer (; from