Why is atomic mass in parentheses?

1 Answer
Aug 1, 2017

The element has no stable isotopes.


For most elements, the atomic mass is not in parentheses, but is a number with at least two decimal places, such as carbon 12.011. If an element has no stable isotopes, meaning that all known isotopes of that element will undergo nuclear decay, the atomic number is displayed in parentheses. For example: neptunium (237).