eli5 why do some elements on the periodic table have atomic weights with decimals and others don’t?

Question

eli5 why do some elements on the periodic table have atomic weights with decimals and others don’t?

In: 3

Answer 1

For most elements, the atomic weight given is the weighted average of all of that element’s isotopes. There is also a small deviation from a whole number in most isotopes because protons and neutrons don’t have exactly the same mass, and because of something called nuclear binding energy.
For radioactive elements, however the given weight is just the mass number (number of protons + number of neutrons) of whichever isotope has the longest half-life.