A chemical element is a pure chemical substance consisting of one type of atom, distinguished by its atomic number, (Figure 1) which is the number of protons in its nucleus. Chemical elements are divided into metals, metalloids, and non-metals. Familiar examples of elements include carbon, oxygen (non-metals), silicon, arsenic (metalloids), aluminium, iron, copper, gold, mercury, and lead (metals).
As of November 2011, 118 elements have been identified, the latest being ununseptium in 2010. Of the 118 known elements, only the first 98 are known to occur naturally on Earth; 80 of them are stable, while the others are radioactive, decaying into lighter elements over various timescales from fractions of a second to billions of years. The elements that do not occur naturally on Earth have been produced artificially as the synthetic products of man-made nuclear reactions.
Hydrogen and helium are by far the most abundant elements in the universe. However, iron is the most abundant element (by mass) making up the Earth, and oxygen is the most common element in the Earth's crust. Although all known chemical matter is composed of these elements, chemical matter itself constitutes only about 15% of the matter in the universe. The remainder is dark matter, a mysterious substance that is not composed of chemical elements since it lacks protons, neutrons, or electrons.
The chemical elements are thought to have been produced by various cosmic processes, including hydrogen and helium (and smaller amounts of lithium, beryllium, and boron ) created during the Big Bang and cosmic-ray spallation. Production of heavier elements, from carbon to the very heaviest elements, proceeds by stellar nucleosynthesis, and these were made available for later solar system and planetary formation by planetary nebulae and supernovae, which blast these elements into space. The high abundance of oxygen, silicon, and iron on Earth reflect their common production in such stars, after the lighter gaseous elements and their compounds have been subtracted. While most elements are generally viewed as stable, a small amount of natural transformation of one element to another also occurs in the present time, through decay of radioactive elements as well as other natural nuclear processes.
Relatively pure samples of isolated elements are uncommon in nature. While all of the 98 naturally occurring elements have been identified in mineral samples from the Earth's crust, only a small minority of elements are found as recognizable, relatively pure minerals. Among the more common of such "native elements" are copper, silver, gold, carbon (as coal, graphite, or diamonds), sulfur, and mercury. All but a few of the most inert elements, such as noble gases and noble metals, are usually found on Earth in chemically combined form, as chemical compounds. While about 32 of the chemical elements occur on Earth in native, uncombined forms, most of these occur as mixtures. For example, atmospheric air is primarily a mixture of nitrogen, oxygen, and argon, and native solid elements occur in alloys, such as that of iron and nickel.
When two distinct elements are chemically combined, with the atoms held together by chemical bonds, the result is termed a chemical compound. Two-thirds of the chemical elements occur on Earth only as compounds, and in the remaining third, often the compound forms of the element are most common. Chemical compounds may be composed of elements combined in exact whole-number ratios of atoms, as in water, table salt, and minerals like quartz, calcite, and some ores. However, chemical bonding of many types of elements results in crystalline solids and metallic alloys for which exact chemical formulas do not exist.
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together in a defined spatial arrangement by chemical bonds. Chemical compounds can be molecular compounds held together by covalent bonds, salts held together by ionic bonds, intermetallic compounds held together by metallic bonds, or complexes held together by coordinate covalent bonds. Pure chemical elements are not considered chemical compounds, even if they consist of molecules that contain only multiple atoms of a single element (such as H2, S8, etc.), which are called diatomic molecules or polyatomic molecules. An example of a chemical compound is water, shown here: Figure 2.