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

Isotopes. Example: Copper is about 69% Cu63, and 31% Cu65 (with miniscule amounts of others. So copper’s atomic weight is .69x 63 + 31%x 65.