Astatine is the penultimate halogen in the periodic table with atomic number 85. It is highly radioactive and would vaporize due to its own radioactivity before being collected.

Wikipedia states that astatine is the rarest naturally occurring element. The total amount of astatine in the Earth's crust (quoted mass [tex]2.36 \times 10^{25}[/tex] grams) is estimated by some to be less than one gram at any given time. Other sources estimate the amount of ephemeral astatine present on Earth at any given moment to be up to one ounce (about 28 grams).

My question is: How can we predict that the amount of astatine on the planet at any time is less than 1 gram or about 28 grams if we haven't had any samples to investigate the element? Are there any indirect means to determine this?

The estimation of the amount of astatine present on Earth at any given time, despite not having any samples to directly investigate the element, is based on indirect means of prediction. Astatine is a highly radioactive element with a very short half-life, so it would quickly decay and disappear. However, scientists can study the decay products of other radioactive elements and calculate the expected abundance of astatine as a result of those decays. This allows them to estimate that the total amount of astatine in the Earth's crust is less than one gram at any given time.