Question

Why do elements such as Neptunium, Plutonium, Californium, Einstenium, Bohrium, Flerovium, Radon, Oganesson, and a few others have half a number in parentheses at the bottom of their box on the periodic table instead of the atomic weight? Thanks! :)

Answer 1

This is because both the protons and the neutrons in an atom have a relatively equal mass

Answer 2

All the elements you listed share a common trait: they are synthetic, radioactive elements. These elements do not have a stable isotope, so for a radioactive element, its most stable isotope's atomic mass will be listed in parentheses.

One such example would be plutonium. This element has 6 different isotopes with differing half-lives. The most stable isotope for plutonium is plutonium-244, with a half-life of around 80 million years. Therefore, the atomic mass listed for plutonium will be (244).