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See also: Elements (table)Elements
Element Symbol Atomic Number Atomic Weight1 Melting Point
(Degrees Celsius)
Boiling Point
(Degrees Celsius)

actinium Ac 89 (227) 1050. 3200. ±300
aluminum Al 13 26.98154 660.37 2467.
americium Am 95 (243) 1172. 2600.
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in chemistry, a substance that cannot be decomposed into simpler substances by chemical means. A substance such as a compoundcompound,
in chemistry, a substance composed of atoms of two or more elements in chemical combination, occurring in a fixed, definite proportion and arranged in a fixed, definite structure. A compound is often represented by its chemical formula.
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 can be decomposed into its constituent elements by means of a chemical reaction, but no further simplification can be achieved. An element can, however, be decomposed into simpler substances, such as protons and neutrons or various combinations of them, by the methods of particle physics, e.g., by bombardment of the nucleus.

The Atom

The smallest unit of a chemical element that has the properties of that element is called an atomatom
[Gr.,=uncuttable (indivisible)], basic unit of matter; more properly, the smallest unit of a chemical element having the properties of that element. Structure of the Atom
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. Many elements (e.g., helium) occur as single atoms. Other elements occur as molecules made up of more than one atom. Elements that ordinarily occur as diatomic molecules include hydrogen, nitrogen, oxygen, and the halogens, but oxygen also occurs as a triatomic form called ozone. Phosphorus usually occurs as a tetratomic molecule, and crystalline sulfur occurs as molecules containing eight atoms.

Atomic Number and Mass Number

Regardless of how many atoms the element is composed of, each atom has the same number of protons in its nucleus, and this is different from the number in the nucleus of any other element. Thus this number, called the atomic number (at. no.), defines the element. For example, the element carbon consists of atoms all with at. no. 6, i.e., all having 6 protons in the nucleus; any atom with at. no. 6 is a carbon atom. By 2016, 118 elements were known, ranging from hydrogen with an at. no. of 1 to oganesson with an at. no. of 118. (See the table entitled ElementsElements
Element Symbol Atomic Number Atomic Weight1 Melting Point
(Degrees Celsius)
Boiling Point
(Degrees Celsius)

actinium Ac 89 (227) 1050. 3200. ±300
aluminum Al 13 26.98154 660.37 2467.
americium Am 95 (243) 1172. 2600.
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 for an alphabetical list of all the elements, including their symbols, atomic numbers, atomic weights, and melting and boiling points.) The nuclei of most atoms also contain neutrons. The total number of protons and neutrons in the nucleus of an atom is called the mass number. For example, the mass number of a carbon atom with 6 protons and 6 neutrons in its nucleus is 12.


Although all atoms of an element have the same number of protons in their nuclei, they may not all have the same number of neutrons. Atoms of an element with the same mass number make up an isotopeisotope
, in chemistry and physics, one of two or more atoms having the same atomic number but differing in atomic weight and mass number. The concept of isotope was introduced by F.
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 of the element. All known elements have isotopes; some have more than others. Hydrogen, for example, has only 3 isotopes, while xenon has 16. Approximately 300 naturally occurring isotopes are known, and more than 2,500 radioactive isotopes have been artificially produced (see synthetic elementssynthetic elements,
in chemistry, radioactive elements that were not discovered occurring in nature but as artificially produced isotopes. They are technetium (at. no. 43), which was the first element to be synthesized, promethium (at. no. 61), astatine (at. no.
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). There are 13 isotopes of carbon, having from 2 to 14 neutrons in the nucleus and therefore mass numbers from 8 to 20.

Not all of the elements have stable isotopes. Some have only radioactive isotopes, which decay to form other isotopes, usually of other elements (see radioactivityradioactivity,
spontaneous disintegration or decay of the nucleus of an atom by emission of particles, usually accompanied by electromagnetic radiation. The energy produced by radioactivity has important military and industrial applications.
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). In some cases all the isotopes of an element are very unstable, and the element is therefore not found in nature. Only 94 of the elements are known to occur naturally on earth. Of these, 6 occur in minute amounts produced by the decay of other elements. These 6 extremely scarce elements and those that do not occur at all naturally were discovered when they were produced in the laboratory; they are often called the man-made, artificially produced, or synthetic elementssynthetic elements,
in chemistry, radioactive elements that were not discovered occurring in nature but as artificially produced isotopes. They are technetium (at. no. 43), which was the first element to be synthesized, promethium (at. no. 61), astatine (at. no.
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Atomic Mass and Atomic Weight

Atoms are not very massive; a carbon atom weighs about 2 × 10−23 grams. Because atoms have so little mass, a unit much smaller than the gram is used. In the current system (adopted in 1960–61) the unit of atomic mass, called atomic mass unitatomic mass unit
or amu,
in chemistry and physics, unit defined as exactly 1-12 the mass of an atom of carbon-12, the isotope of carbon with six protons and six neutrons in its nucleus. One amu is equal to approximately 1.66 × 10−24 grams.
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 (amu), is defined as exactly 1-12 the mass of an atom of carbon-12. The atomic weightatomic weight,
mean (weighted average) of the masses of all the naturally occurring isotopes of a chemical element, as contrasted with atomic mass, which is the mass of any individual isotope. Although the first atomic weights were calculated at the beginning of the 19th cent.
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 of an element is the mean (weighted average) of the atomic masses of all the naturally occurring isotopes. Carbon has two principal naturally occurring isotopes, carbon-12 and carbon-13. Carbon-12, whose mass is defined as exactly 12 amu, constitutes 98.89% of naturally occurring carbon; carbon-13, whose mass is 13.00335 amu, constitutes 1.11%. (There are also small traces of the radioactive isotope carbon-14.) The atomic weight of the element is determined by multiplying the percent abundance of each isotope by the atomic mass of the isotope, adding these products, and dividing by 100. However, isotope abundance is often determined by the medium of the source, solid, liquid, or gas, and the average atomic weight may fluctuate. Thus, for carbon, [(98.89 × 12.000) + (1.11 × 13.00335)]/100 = 12.01115, which is the atomic weight of the element carbon in amu, but because the proportions of the isotopes vary depending on where the carbon is found, carbon's atomic weight is now expressed as an interval defined by the lower and upper bounds within which the atomic weight ranges: [12.0096; 12.0116]. Certain synthetic elements exist only momentarily in the form of a few short-lived isotopes; in such cases the concept of atomic weight cannot be applied.

Properties of the Elements

Properties of an element are sometimes classed as either chemical or physical. Chemical properties are usually observed in the course of a chemical reaction, while physical properties are observed by examining a sample of the pure element. The chemical properties of an element are due to the distribution of electrons around the atom's nucleus, particularly the outer, or valence, electrons; it is these electrons that are involved in chemical reactions. A chemical reaction does not affect the atomic nucleus; the atomic number therefore remains unchanged in a chemical reaction.

Some properties of an element can be observed only in a collection of atoms or molecules of the element. These properties include color, density, melting point, boiling point, and thermal and electrical conductivity. While some of these properties are due chiefly to the electronic structure of the element, others are more closely related to properties of the nucleus, e.g., mass number.

The elements are sometimes grouped according to their properties. One major classification of the elements is as metalsmetal,
chemical element displaying certain properties by which it is normally distinguished from a nonmetal, notably its metallic luster, the capacity to lose electrons and form a positive ion, and the ability to conduct heat and electricity.
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, nonmetalsnonmetal,
chemical element possessing certain properties by which it is distinguished from a metal. In general, this distinction is drawn on the basis that a nonmetal tends to accept electrons and form negative ions and that its oxide is acidic.
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, and metalloids. Elements with very similar chemical properties are often referred to as families; some families of elements include the halogens, the inert gases, and the alkali metals. In the periodic tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
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 the elements are arranged in order of increasing atomic weight in such a way that the elements in any column have similar properties.

Official Symbols and Names for the Elements

Each element is assigned an official symbol by the International Union of Pure and Applied Chemistry (IUPAC). For example, the symbol for carbon is C, and the symbol for silver is Ag [Lat. argentum = silver]. There are several ways of designating an isotope. One designation consists of the name or symbol of the element followed by a hyphen and the mass number of the isotope; thus the isotope of carbon with mass number 12 can be designated carbon-12 or C-12. The mass number is often written as a superscript, e.g., C12; sometimes the atomic number is written as a subscript preceding the symbol, e.g., 6C12. The IUPAC rules for nomenclature of inorganic chemistry state that the subscript atomic number and superscript mass number should both precede the symbol, e.g.,    126C.

Many isotopes were given special names and symbols when they were first discovered in natural radioactive decay series (e.g., uranium-235 was called actinouranium and represented by the symbol AcU). This practice is discouraged in the modern nomenclature except in the case of hydrogen. The isotopes hydrogen-2 and hydrogen-3 are usually called deuterium and tritium, respectively. Hydrogen-1, the most abundant isotope, has the name protium but is usually simply called hydrogen. Newly discovered elements that have been synthesized by one laboratory and not yet confirmed by a second are given a provisional name based on Greek and Latin roots; when the discovery is confirmed, the laboratory that first made it may suggest a name for the element.

The Elements through the Ages

Some elements have been known since antiquity. Gold ornaments from the Neolithic period have been discovered. Gold, iron, copper, lead, silver, and tin were used in Egypt and Mesopotamia before 3000 B.C. However, recognition of these metals as chemical elements did not occur until modern times.

Greek Concept of the Elements

The Greek philosophers proposed that there are basic substances from which all things are made. EmpedoclesEmpedocles
, c.495–c.435 B.C., Greek philosopher, b. Acragas (present Agrigento), Sicily. Leader of the democratic faction in his native city, he was offered the crown, which he refused. A turn in political fortunes drove him and his followers into exile.
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 proposed four basic "roots," earth, air, fire, and water, and two forces, harmony and discord, joining and separating them. PlatoPlato
, 427?–347 B.C., Greek philosopher. Plato's teachings have been among the most influential in the history of Western civilization. Life

After pursuing the liberal studies of his day, he became in 407 B.C. a pupil and friend of Socrates. From about 388 B.
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 called the roots stoicheia (elements). He thought that they assume geometric forms and are made up of some more basic but undefined matter. A different theory, that of LeucippusLeucippus
, 5th cent. B.C., Greek philosopher. Aristotle believed that Leucippus inspired the atomistic theory with which Democritus is identified. Little is known about Leucippus.
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 and his followers, held that all matter is made up of tiny indivisible particles (atomos).

This theory was rejected by AristotleAristotle
, 384–322 B.C., Greek philosopher, b. Stagira. He is sometimes called the Stagirite. Life

Aristotle's father, Nicomachus, was a noted physician. Aristotle studied (367–347 B.C.
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, who expanded on Plato's theory. Aristotle believed that different forms (eidos) were assumed by a basic material, which he called hulé. The hulé had four basic properties, hotness, coldness, dryness, and moistness. The four elements differ in their embodiment of these properties; fire is hot and dry, earth cold and dry, water cold and moist, and air hot and moist. Although Aristotle proposed that an element is "one of those simple bodies into which other bodies can be decomposed and which itself is not capable of being divided into others," he thought the metals to be made of water, and called mercury "silver water" (chutos arguros). His idea that matter was a single basic substance that assumed different forms led to attempts by the alchemists to transmute other metals into gold.

Evolution of Modern Concepts

Although much early work was done in chemistry, especially with metals, and many recipes were recorded, there were few developments in the conception of the elements. In the 16th cent. ParacelsusParacelsus, Philippus Aureolus
, 1493?–1541, Swiss physician and alchemist. His original name Theophrastus Bombastus von Hohenheim. He traveled widely, acquiring knowledge of alchemy, chemistry, and metallurgy, and although his egotism and his contempt for traditional
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 proposed salt, mercury, and sulfur as three "principles" of which bodies were made, although he apparently also believed in the four "elements." Van Helmont (c.1600) rejected the four elements and three principles, substituting two elements, air and water.

Robert BoyleBoyle, Robert,
1627–91, Anglo-Irish physicist and chemist. The seventh son of the 1st earl of Cork, he was educated at Eton and on the Continent and conducted most of his researches at his own laboratories at Oxford (1654–68) and London (1668–91).
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 rejected these early theories and proposed a definition of chemical elements that led to the currently accepted definition. His definition is strikingly similar to Aristotle's earlier definition. In The Sceptical Chymist (1661) Boyle wrote, "I now mean by elements … certain primitive and simple, or perfectly unmingled bodies; which not being made of any other bodies, or of one another, are the ingredients of which all those called perfectly mixed bodies [chemical compounds] are immediately compounded, and into which they are ultimately resolved."

Whereas Aristotle and other early philosophers tried to determine the identity of the elements solely by reason, Boyle and later scientists used the results of numerous experiments to identify the elements. In 1789 Antoine LavoisierLavoisier, Antoine Laurent
, 1743–94, French chemist and physicist, a founder of modern chemistry. He studied under eminent men of his day, won early recognition, and was admitted to the Academy of Sciences in 1768.
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 published a list of chemical elements based on Boyle's definition; this encouraged adoption of standard names for the elements. Although some of his elements are now known to be compounds, such as metallic oxides and salts, they were at the time accepted as elements since they could not be decomposed by any method then known.

In 1803 John DaltonDalton, John
, 1766–1844, English scientist. He revived the atomic theory (see atom), which he formulated in the first volume of his New System of Chemical Philosophy (2 vol., 1808–27).
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 proposed (as part of his atomic theory) that all atoms of an element have identical properties (including mass), that these atoms are unchanged by chemical action, and that atoms of different elements react with one another in simple proportions. Although symbols for some of the elements already existed, they were by no means universally accepted, and each compound also had a unique symbol that was unrelated to its chemical composition. Dalton devised a new set of circular symbols for the elements and used a combination of elemental symbols to represent a compound. For example, his symbol for oxygen was ◯, and for hydrogen ⊙. Since he thought water contained one atom of hydrogen for every atom of oxygen, he formed the symbol for water by writing the symbols for hydrogen and oxygen touching one another, ⊙◯. J. J. BerzeliusBerzelius, Jöns Jakob, Baron
, 1779–1848, Swedish chemist, M.D. Univ. of Uppsala, 1802. He was noted for his work as teacher at the medical school and other institutions in Stockholm and for his discoveries in diverse fields of chemistry.
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 was the first to use the modern method, letting one or two letters of the element's name serve as its symbol. He also published an early table of atomic weights of 24 elements with most values very close to those now in use.

Discovery of the Elements

As noted above, some of the elements were discovered in prehistoric times but were not recognized as elements. Arsenic was discovered around 1250 by Albertus MagnusAlbertus Magnus, Saint
, or Saint Albert the Great,
b. 1193 or 1206, d. 1280, scholastic philosopher, Doctor of the Church, called the Universal Doctor. A nobleman of Bollstädt in Swabia, he joined (1223) the Dominicans and taught at Hildesheim, Freiburg,
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, and phosphorus was discovered about 1674 by Hennig Brand, an alchemist, who prepared it by distilling human urine. Only 12 elements were known before 1700, and only about twice that many by 1800, but by 1900 over 80 elements had been identified. In 1919 Ernest RutherfordRutherford, Ernest Rutherford, 1st Baron,
1871–1937, British physicist, b. New Zealand. Rutherford left New Zealand in 1895, having earned three degrees from the Univ.
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 found that hydrogen was given off when nitrogen was bombarded with alpha particles. This first transmutation encouraged further study of nuclear reactions, and eventually led to the discovery in 1937 of technetium, the first synthetic element. Neptunium (atomic number 93) was the first transuranium elementtransuranium elements,
in chemistry, radioactive elements with atomic numbers greater than that of uranium (at. no. 92). All the transuranium elements of the actinide series were discovered as synthetic radioactive isotopes at the Univ.
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 to be synthesized (1940). Its discovery prompted the search that has led to the ongoing synthesis of additional transuranium elements.


See J. Emsley, The Elements (1991); A. Swertka, A Guide to the Elements (1996); P. W. Atkins, The Periodic Kingdom (1997); N. N. Greenwood and A. Earnshaw, Chemistry of the Elements (2d ed. 1997).


(el -ĕ-mĕnt) Any of a large number of substances, including hydrogen, helium, carbon, nitrogen, oxygen, and iron, that consist entirely of atoms of the same atomic number, i.e. with the same number of protons in their nuclei. The atoms are not all identical however: isotopes of an element can occur with different numbers of neutrons in their nuclei. When arranged in order of increasing atomic number along a series of horizontal rows, elements with similar chemical and physical properties fall into groups. The similarities in properties arise from similarities in the electron arrangements within the atom. This table of elements is called the periodic table. Over 100 elements are now known, more than 90 of which occur naturally on Earth. The elements have stable isotopes and/or unstable (i.e. radioactive) isotopes.

The creation of the elements and the means by which they become distributed throughout the Universe are major areas of study in astronomy (see nucleosynthesis). The abundance of each element in the Universe, i.e. its cosmic abundance, depends on the method of its synthesis – by nuclear reactions in stars, by cosmic-ray collisions, etc. – and on its lifetime in its immediate surroundings and its long-term stability.


An integral part of the sub- or superstructure having its own functional requirements; such as foundations, walls, floors, roofs, stairs and structural framework.



(1) Among ancient Greek materialist philosophers, the basic constituents of nature; Empedocles believed the four basic elements to be fire, air, water, and earth. In ancient Chinese philosophy, the basic elements were thought to be metal, earth, water, wood, and fire.

(2) In the plural, a phenomenon or force of nature, such as a storm, that is regarded as irrepressible.

(3) In a figurative sense, one’s habitual environment or surroundings; a favorite, familiar occupation or pursuit.


A substance made up of atoms with the same atomic number; common examples are hydrogen, gold, and iron. Also known as chemical element.
(computer science)
A circuit or device performing some specific elementary data-processing function.
A part of an electron tube, semiconductor device, or antenna array that contributes directly to the electrical performance.
Radiator, active or parasitic, that is a part of an antenna.
(industrial engineering)
A brief, relatively homogeneous part of a work cycle that can be described and identified.
In an array such as a matrix or determinant, a quantity identified by the intersection of a given row or column.
In network topology, an edge.
The generatrix of a ruled surface at any one fixed position.
(civil engineering)

building element

An architectural component of a building, facility, or site.


1. any of the 118 known substances (of which 93 occur naturally) that consist of atoms with the same number of protons in their nuclei
2. the most favourable environment for an animal or plant
3. the resistance wire and its former that constitute the electrical heater in a cooker, heater, etc.
4. Electronics another name for component
5. one of the four substances thought in ancient and medieval cosmology to constitute the universe (earth, air, water, or fire)
6. atmospheric conditions or forces, esp wind, rain, and cold
7. Geometry a point, line, plane, or part of a geometric figure
8. Maths
a. any of the terms in a determinant or matrix
b. one of the infinitesimally small quantities summed by an integral, often represented by the expression following the integral sign
9. Maths Logic one of the objects or numbers that together constitute a set
10. Christianity the bread or wine consecrated in the Eucharist
11. Astronomy any of the numerical quantities, such as the major axis or eccentricity, used in describing the orbit of a planet, satellite, etc.
12. Physics a component of a compound lens


(data, programming)
One of the items of data in an array.


(language, text)
One kind of node in an SGML, HTML, or XML document tree. An SGML element is typically represented by a start tag ("<p>") and an end tag ("</p>"). In some SGML implementations, some tags are omissible, as with "</p>" in HTML.

The start tag can contain attributes ("<p lang="en-UK" class='stuff'>"), which are an unordered set of key-value bindings for that element. Both the start tag and end tag for an element typically contain the "tag name" (also called the "GI" or generic identifier) for that element.

In XML, an element is always represented either by an explicit start tag and end tag, or by an empty element tag ("<img src='thing.png' alt='a dodad' />").

Other kinds of SGML node are: a section of character data ("foo"), a comment ("<!-- bar -->"), a markup declaration ("<!ENTITY reg CDATA '®'>"), or a processing instruction ("<?xml-stylesheet href="shop-english.xsl" type="text/xsl" ?>").
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