February 27th: Riddle #2

Guess the element and the isotope!

*isotope - a version of an element with specific mass number

Resource: shutterstock.

I am very rare😱, about one part in trillion atoms of my element.

(Almost like 1 second in 31,500 years!)

I am radioactive☢️, though small.

I am found in all living organisms, which, while alive, keep my proportion within them the same🐒🐟🦅.

When they die, I start to dwindle away🫥.

Sharp-witted humans learnt to count me up😎, and now they can discern the age of anything that was once alive🕰️.

What am I?🤔

[Click to view answer here]