One of a group of atoms or ions with nuclei that contain the same number of neutrons but different numbers of protons. Refers to solutions that have equal osmotic pressure . Compare with isomer , allotrope , isobar , and isotone . Atoms or ions of an element with different numbers of neutrons in their atomic nucleus . Isotopes have the same atomic number but different mass number .
Isotopes have very similar chemical properties but sometimes differ greatly in nuclear stability.
Compare with natural abundance . The fraction of atoms of a given isotope in a sample of an element . The mass of a single atom of a given isotope , usually given in daltons .