system
Given the following post, respond as if you're commenting on Reddit:
human
Why cant elements with atomic numbers above 94 be found in nature?

I thought it had to do with the half life of those elements but Curium 248 has a relatively long half life. Longer than Neptunium isotopes which can be found in trace amounts so why?
gpt
There's no law of physics that says they *can't* be found in nature. What is and isn't found in nature is set by two things:

1. What natural processes are available to produce it?

2. If it decays, what is its half-life?

For nuclides with half-lives significantly less than the Earth, even if they were produced in large amounts when the solar system formed, they will have mostly decayed away by now, and therefore won't be common in nature (some nuclides are continually produced by natural processes, and so they do exist in some amount naturally despite having short half-lives).

In the example you give of curium-248 (and actually curium-247 is a better example, because it's got a longer half-life), the half-life is "long" in human terms. But the Earth is a little under 5 billion years old, and the half-life in this case is on the order of 10^(5) years (or 10^(7) years for curium-247). That's still very small compared to the age of the Earth, which is on the order of 10^(9) years. So whatever amount of curium that was produced in the same astrophysical event(s) as the rest of our solar system will have decayed away by now. And there apparently aren't any ongoing natural processes which produce it in at least "trace" amounts (for whatever definition of "trace" you're assuming in the question).