Elements that belong to the same group usually show chemical similarities, although the element at the top of the group is usually atypical. An atom or molecule that has acquired a charge by either gaining or losing electrons. 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 . There is no change in total mass during a chemical change. The demonstration of conservation of mass by Antoine Lavoisier in the late 18th century was a milestone in the development of modern chemistry.