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1) In chemistry and physics, an element is a substance that cannot be broken down into simpler components by any non-nuclear chemical reaction. An element is uniquely determined by the number of protons in the nuclei of its atoms. The number of protons in the nucleus of the atom of an element is known as the atomic number of that element.

The number of electrons in the atom of a specific element can vary. If the number of electrons is the same as the number of protons, then the atom has no electric charge. However, if the number of electrons is different from the number of protons, the atom is called an ion. If there are more electrons than protons, it is a negative ion; if there are fewer electrons than protons, it is a positive ion.

The number of neutrons in the atom of a specific element can also vary. The number of neutrons in the nucleus of an element determines the isotope of the element. All elements have a particular isotope that is most common in nature. For example, the most common isotope of hydrogen has no neutrons, but isotopes exist with 1 neutron (called deuterium) and 2 neutrons (called tritium). The most common naturally occurring isotope of carbon has 6 neutrons in the nucleus, but another, less common isotope has 8.

Some elements can be broken down into elements with smaller atomic numbers by so-called atomic or nuclear reactions. This process is called atomic fission or nuclear fission, and was responsible for the unprecedented explosions of the first atom bombs (equivalent to thousands of tons of TNT). It is also the process by which nuclear reactors generate power.

Sometimes elements combine to form other elements with larger atomic numbers. This is called atomic fusion or nuclear fusion. The most common nuclear fusion process involves the conversion of hydrogen to helium. It occurs in the core of the Sun and other stars. Nuclear fusion produces more energy per unit mass than fission, and is responsible for the horrific destructive power of the hydrogen bomb (in some cases equivalent to more than 20 million tons of TNT).

In nature, elements are found with atomic numbers from 1 through 92. Hydrogen, the most abundant element in the universe, has atomic number 1. Helium, the second most abundant element in the universe, has atomic number 2. Other familiar elements include carbon with 6 protons in its nucleus, nitrogen with 7, oxygen with 8, iron with 26, and uranium with 92. Elements with atomic numbers greater than 92 have been made in laboratories, but most of them tend to be unstable and decay rapidly into elements with lower atomic numbers.

2) In mathematical set theory, an element is a member of a set. The condition in which an object is an element of a set is symbolized . For example, if A denotes the set {1, 2, 3, 4}, then 3 A. The condition in which an object is not an element of a set is symbolized . Thus, in the foregoing scenario, 7 A. An entry in a matrix is sometimes called an element. So is the expression following an integral sign.

This was last updated in May 2008
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