What is the rarest element?

Hint:Elements are defined as the simple chemical substance that consists of the same atoms. In chemistry, we study the chemical properties and physical properties of these elements and compounds. Elements consist of only one type of atom which is arranged on the periodic table. They are arranged on the basis of metallic and non metallic behaviour.

Complete step-by-step answer:Elements are defined as a species in which there are the same numbers of protons in their atomic nuclei. The examples of elements are Nitrogen, Oxygen, Sodium, Magnesium, aluminium, hydrogen, carbon, potassium, calcium and so on.
The rarest element on earth is Astatine which is obtained as the decay product of heavier elements. The symbol of Astatine is $At$ which has an atomic number $85$ . All the isotopes of Astatine are short lived and the most stable isotope is Astatine $-210$ . It has a half life of $8.1$ hours. It is present in period six and group seventeen. It shows face centered cubic structure.
Let us see some facts of Astatine-
Astatine is a decay product of Uranium and Thorium.
The isotopes of astatine are bismuth $-206$ and polonium $-210$ and the formation of these isotopes depends upon the process of decay.
It is so rare that only $25g$ of naturally occurring Astatine is present in the earth crust.
Astatines show similar properties like halogen, that is they are non metals, and show low melting and boiling point.
Astatine is so radioactive still the environmental and health hazard is zero because they are rare elements.

Note:It is to note that the current research says that Astatine is helpful in treating cancer in the thyroid gland. Astatine goes to the thyroid gland and kills cancer cells with the help of their radiation. It is the heaviest known halogen. To study Astatine, we require special handling techniques and precautions.