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Chemical elements

A chemical element is any substance that cannot be decomposed into simpler substances by ordinary chemical processes. Chemical elements constitute the fundamental materials of which all matter consists. Of all the known elements, 90 occur in nature either chemically free or in combination with other elements.

About one-third of the elements found in nature occur in a chemically free state on Earth. These elements, which obviously are not very active chemically, include nitrogen, gold, platinum, copper, and the noble gases. The five most abundant elements in the Earth’s crust are oxygen (466, 000 parts per 1, 000, 000, or 46.6 percent), silicon (27.7 percent), aluminum (8.1 percent), iron (5 percent), and calcium (3.6 percent). Hydrogen is by far the most abundant element in the universe, accounting for more than 90 percent of the total number of atoms and for about three-fourths of the mass. Helium is next in abundance, constituting about 7 percent of the number of atoms and nearly one-fourth of the total mass.

The properties of the elements are to a large degree attributable to the electronic structure and size of their atoms. Accordingly, they are extremely diverse. Helium, for example, has the lowest-known melting point (-271.4 ⁰C at 29.6 atmospheric pressure) and boiling point (-268.98 ⁰C) of any of the elements, while tungsten has the highest-known melting point (3, 370 ⁰C) and boiling point (5, 900 ⁰C).

The atomic number of an element indicates the number of protons (positively charged particles) in the nucleus of a given atom. It is also the number of electrons (negatively charged particles) in the atom that determines the chemical properties of the element. Each electron moves in an atomic orbit (equivalent to a particular energy level) around the nucleus.

The ability of an atom to combine chemically with another atom is called its valence. The valence of an element is the number of electrons it needs to gain or lose in order to make it stable. Atoms require a total of eight electrons in the outermost energy level to be stable in the case of the lightest atoms (hydrogen and helium) the number is two.

The way in which atoms are held together is known as bonding. The covalent bond is a type of chemical bond in which atoms are held together by shared pairs of electrons which move around both atoms. The electrovalent (or ionic) bond is another kind of chemical bond between atoms. Here, electrons are transferred from one atom to another forming ions. This produces electrostatic attraction; in other words, the attraction between opposite charges holds the ions together.

As the list of different elements grew in the 19^th century–by 1860 over 80 were known – chemists attempted to group them together and to classify them according to their behavior. In 1869 the Russian chemist Dmitri Ivanovich Mendeleyev made a complete table by arranging the elements in order of their atomic weights in rows so that elements with similar properties appeared in columns.

The Periodic Table produced by Mendeleyev was refined by further research into the structure of atoms. At present there are 110 known chemical elements. (The discovery of element 110, reported in 1987 by Soviet scientists, remains unconfirmed, however, and is still considered extremely tentative).