Some element categories in the periodic table The element does not have any stable nuclides, and a value in brackets, e.g. [209], indicates the mass number of the longest-lived isotope of the element. However, three elements, thorium, protactinium, and uranium, have a characteristic terrestrial isotopic composition, and thus their atomic mass given. The isotopic composition of this element varies in some geological specimens, and the variation may exceed the uncertainty stated in the table. The isotopic composition varies in terrestrial material such that a more precise atomic weight can not be given. The isotopic composition of the element can vary in commercial materials, which can cause the atomic weight to deviate significantly from the given value. The atomic weight of commercial Lithium can vary between 6.939 and 6.996—analysis of the specific material is necessary to find a more accurate value.