What is the rarest single element in the Earth's crust of those named in table 1?

The rarest single element in the Earth's crust is typically considered to be francium. It is extremely unstable and radioactive, with an estimated abundance of only about 30 grams in the entire Earth's crust at any given time. Due to its short half-life and the fact that it decays rapidly, francium is rarely encountered in nature, making it the least abundant naturally occurring element.