On the periodic table, the atomic mass of an element is typically listed as a decimal number without parentheses. The decimal number represents the average atomic mass of all the naturally occurring isotopes of that element, taking into account their relative abundance. However, for elements with a significant number of unstable or radioactive isotopes, it is common to list the atomic mass of the most stable or most abundant isotope without parentheses.
Protactinium (element 91) is one such element that has a relatively unstable and radioactive isotope, protactinium-231 (231Pa), which undergoes radioactive decay. This isotope has a long half-life but is not present in significant quantities in nature. Instead, protactinium occurs as a mixture of several isotopes, including protactinium-231 and protactinium-233 (233Pa). Due to the presence of these isotopes with different atomic masses and their relative abundances, it is more appropriate to report the atomic mass of protactinium as a range rather than a single value.
The atomic mass of protactinium is usually given as [231.03588, 231.03599] on the periodic table, indicating the range of atomic masses for the isotopes found in nature. This notation helps to convey that the atomic mass can vary slightly depending on the sample.