A vertical column on the periodic table, for example, the halogens . 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.