Besides Technetium and Promethium, my periodic table says everything before Uranium (Atomic #92) is found in nature (on earth). Everything after isn’t. I figured since Promethium is in the Lanthanoid series, it isn’t unusual that it isn’t in nature (since most of them are rare). But Technetium is between Molybdenum and Ruthenium, which are a lot more common.
So is there a reason there isn’t any (with few exceptions dealing with radiation) on earth?
And the atomic number probably has very little to do with it’s rarity I know, but my point is that there are only two exceptions and I’d like to know why there are.