One of the 114 presently known kinds of substances that constitute all matter at and above the level of atom s (the smallest units of any element).
All atoms of an element are identical in nuclear charge (number of proton s) and number of electron s (see atomic number ), but their mass ( atomic weight ) may differ if they have different numbers of neutron s (see isotope ). Each element has a one-or two-letter chemical symbol . Elements combine to form a wide variety of compound s. All elements with atomic numbers greater than 83 (bismuth), and some isotopes of lighter elements, are unstable and radioactive (see radioactivity ). The transuranium element s, with atomic numbers greater than 92 (see uranium ), artificially created by bombardment of other elements with neutrons or other particles, were discovered beginning in 1940. The most common elements (by weight) in Earth's crust are oxygen, 49%; silicon, 26%; aluminum, 8%; and iron, 5%. Of the known elements, 11 (hydrogen, nitrogen, oxygen, fluorine, chlorine, and the six noble gases) are gas es under ordinary conditions, two (bromine and mercury) are liquid s (two more, cesium and gallium, melt at about or just above room temperature), and the rest are solid s. See also periodic table .