As the number of protons and neutrons in the nucleus increases, elements are much less likely to form stable, non-radioactive isotopes. of elements that form non-radioactive isotopes, which has the highest atomic number?

The answer is iron. Iron has an atomic number of 26. This means than iron has 26 protons in it's nucleus. However, not all iron atoms have the same number of neutrons. Most commonly iron has 30 neutrons, but it also has 3 other stable isotopes. The word stable is important here because unstable elements are radioactive.

Moles is the unit of measurement for amount of substance in the International System of Units.