I've read that Francium is so rare that no more than 30g Exists on earth at any one time. What makes Francium exist in much lesser quantities than some of the other alkali metals?

It's radioactive — there are no stable isotopes of it. And the longest-lived isotopes have half-lives of order 20 minutes, so even if you had something producing it in a decay chain, it will decay way pretty quickly.

It's radioactive — there are no stable isotopes of it. And the longest-lived isotopes have half-lives of order 20 minutes, so even if you had something producing it in a decay chain, it will decay way pretty quickly.

 

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