A heavy element is an element with an atomic number greater than 20. The term “heavy element” is used in various fields of science, including chemistry, astrophysics, and nuclear physics.
In chemistry, a heavy element is an element that has an atomic number greater than 20. The term is used in contrast to “light elements”, which have atomic numbers less than 20. Heavy elements are generally characterized by their high density and low volatility. Many of the properties of heavy elements are due to the strong forces between the electrons in their outermost shells.
Heavy elements are found in all three phases of matter: solid, liquid, and gas. In solids, they are found either in the form of atoms or molecules. In liquids, they exist as ions in solution. In gases, they exist as atoms or molecules. Some heavy elements (such as uranium) are radioactive and will decay over time into lighter elements (such as lead).
The heaviest naturally-occurring element is uranium, with an atomic number of 92. The second heaviest natural element is thorium, with an atomic number of 90. Both uranium and thorium are found in trace amounts in the Earth’s crust and mantle; however, most of these elements were created synthetically through nuclear reactions (see below).
The heaviest known man-made element is plutonium-244 (atomic number 94), although it is only produced in very small quantities via neutron capture from americium-241 (atomic number 95). Plutonium-244 has a half-life of about 80 million years—much longer than any other known man-made isotope—making it suitable for use in long-lived radioactive waste disposal schemes such as deep geological burial. Other transactinides—elements with atomic numbers greater than 103—have been synthesized in laboratories; however, none have yet been observed to occur naturally on Earth.
Although most heavy elements are synthetic radioisotopes with relatively short half-lives, some nonradioactive stable heavy elemental isotopes do occur naturally on Earth: lead (Pb) with four stable isotopes comprising almost all terrestrial lead; bismuth (Bi) with two stable isotopes comprising all terrestrial bismuth; and mercury (Hg) which has only one stable isotope but this comprises all terrestrial mercury except for a tiny amount present as Hg–202 resulting from cosmic ray bombardment.”