A chemical element (or element) is a chemical substance consisting of atoms having the same number of protons in their atomic nuclei (atomic number or Z number).[1] One hundred eighteen elements have been identified, of which the first 98 occur naturally on Earth. Eighty elements have stable isotopes, while the others have only radioactive isotopes, and in time decay into other elements. Iron is the most abundant element (by mass) making up the Earth, while oxygen is the most common element in the crust of the earth