It is estimated that there is no more than 30 grams of this element in the earth's crust at any one time.

Astatine is the limited element found in the Earth's crust. This element is the rarest one found in the crust and it is radioactive. It is named after the Greek word "astatos" which means unstable. The first acknowledgement of it, came in 1940, which was when it was declared.

If the heat engine is a perfect carnot cycle engine, thenefficiency = (t2-t1)/t2   where t1=temperature of cold reservoir ( ° k) = 135            t2=temperature of hot reservoir ( ° k)   = 325thereforeefficiency = (t2-t1)/t2 = (325-135)/325 = 0.5846 = 58%  note: this is the theoretical maximum efficiency of a perfect heat engine.   real-life engines do not achieve this value.