Originally Posted by Mutt

This was a man-made element. How do we know we've found all of the naturally occurring elements?

The good thing with elements is that they're defined by atomic numbers, meaning they're defined by the number of protons in the nucleus. This number is never a fraction, so you can't have, say, 3.2 protons in a nucleus. So we know we have them all because we know of an element with one proton and an element with two protons and so on.

This scientist is saying they know they have all the natural elements, which means that any new element won't be 'found', it will be made in a laboratory.