Chemical elements

Familiar chemical elements include oxygen, copper, gold, and mercury &mdash; among 94 naturally occurring chemical elements on Earth, listed in tables below.

"Chemical elements", in one conceptual sense, refers to species, or types, of atoms. The distinguishing characteristic of a species of atoms is the number of units of positive charge (i.e., number of protons, each carrying a unit of positive charge) in the nucleus of its atoms, referred to as the atomic number. That is, the atomic number is unique for each species of atoms and therefore for each corresponding chemical element. The chemical elements, oxygen, copper, gold, and mercury, and all the other chemical elements, thus each have a unique atomic number, specified in the tables below. In summary, chemists conceive of oxygen and the other chemical elements as species of atoms, defined by their atomic numbers.

In a second common conceptual sense, "chemical elements" refers to chemically pure substances each composed of a collection of many atoms, of a single species, or type, of atom, again as distinguished by its atomic number. In this sense of a chemical element, sometimes the term "elementary substance" is used, but most often chemical element is used for both senses. Familiar examples of such pure chemical substances &mdash; of such elementary substances &mdash; are segments of electrical current-conducting wire made solely of copper atoms, and handy rolls of flexible kitchen foil made solely of aluminum atoms. This second sense, or concept, makes the term chemical element somewhat more tangible, more evident to direct sense perception, by defining it in terms of a substance, implying perceptual tangibility. Though chemical elements can be conceptualized as pure chemical substances, and produced as such by humans by a variety of special methods, in nature most chemical elements occur in association with other chemical elements. Aluminum atoms, for example, mentioned above, in nature always appear bound to atoms of other species, most commonly to oxygen atoms.

Historical note regarding the definition of "chemical element"
Prior to John Dalton's development and advocation of a quantitative atomic theory at the turn of the 19th century, and prior to the consensus on definition reached in the 20th century, a chemical element had been defined as a substance that ordinary chemical methods could not decompose into simpler substances. Some introductory chemistry textbooks still give that older definition as primary, as does the current (2009) edition of the Encyclopedia Britannica Online. Since one cannot know whether future technology will provide a chemical method to further simplify a presumed 'pure' substance, that older concept cannot be definitive.

A quote from Per Enghag's Encyclopedia of the Elements gives a concise recapitulation:

In 19th-century textbooks of chemistry, elements were defined as simple bodies, which cannot be divided into other different elements by available means. This definition is still valid if available means are simple chemical or electrochemical reactions. Thus, water is not an element because it can be split into the elements hydrogen and oxygen. Further dividing is not possible by simple means.

The modern definitions of chemical elements described in the introduction to this article do not accord with the implication of the older conception of a chemical element as a substance having identical component parts (simple bodies), as methods have been developed, though not chemical methods, that can separate a species of chemical element into 'varieties' called isotopes, all with the same number of protons in the nucleus, and therefore accord with the modern definitions, but differing in numbers of uncharged particles, neutrons, in the nucleus, hence differing in atomic weight. See the section on isotopes later in this article for the implications of an atomic species characterized by admixtures of atoms with differing structures and weights.

Elementary facts about chemical elements
The atomic number of a chemical element, indicating the number of units of positive charge, or number of protons, in the atomic nucleus, is symbolized by the letter Z. Among the 94 naturally occurring chemical elements, the atoms of the element hydrogen, Z=1, have the fewest number of protons, and those of the element plutonium, Z=94, have the greatest number of protons. As protons each carry a positive charge, Z gives the positive charge of the nucleus in units of the so-called elementary charge, symbolized e. It is known that Z electrons (of charge &minus;e, or negative e, and of mass much smaller than the proton) "orbit" the nucleus of an atom, so that an atom as a whole is electrically neutral, with its mass concentrated in the nucleus.

The first 94 elements occur naturally on Earth, although the radioactive elements technetium, Z=43, promethium Z=61, astatine, Z=85, francium, Z=87, neptunium, Z=93, and plutonium, Z=94  are extremely rare and are mainly man-made. The elements beyond Z=94 do not occur naturally on Earth, they are all man-made and radioactive. Non-radioactive elements are stable, and will live as long as the universe, while the radioactive elements,  have finite life times and decay into other elements while emitting radiation. The so-called "transuranic elements" run from Z = 93 to 118. The elements with are sometimes referred to as "cisuranic". Some non-radioactive elements, such as the gas neon, are also very rare on Earth.

The names of the elements are of historical origin and may differ among natural languages. The atomic number (Z), on the other hand, is universally the unique designator of an element, as is its international chemical symbol consisting of one or two letters.

People from all walks of everyday life know something about many different chemical elements, even if they do not recognize them as such. They include: helium (He), used to make party balloons float; lithium (Li), used to make batteries for laptops and cellphones, and in some medication; oxygen (O), in the air we breathe; neon (Ne), in 'neon' lights; sodium (Na), which is present in the table salt that nutritionists advise using sparingly in foods; aluminum (Al), used as foil for wrapping leftovers and basting turkeys; silicon (Si), used to make computer chips; sulfur, the sulfur pools in Hawaii, the sulfuric acid in car batteries; chlorine (Cl), used to make household bleach; potassium, foods rich in potassium touted on TV for cardiovascular health; calcium (Ca), people take supplements to have healthy bones, milk known as rich dietary source; iron (Fe), present in blood and used for many tools; nickel, used in coins; copper, used in electrical and telephone wires, and copper pots and pans; arsenic (As), used as a poison, problems reported on TV with arsenic-contaminated water; silver (Ag), used in jewelry, coins and tableware.

All matter directly perceptible by the human senses &mdash; whether solid, liquid or gas &mdash; is composed of one or more elements. Typically, elements are found in nature in the form of a collection of atoms, often with the atoms of other elements, as compounds (e.g., iron ore, a collection of unit compounds each of iron and oxygen atoms, oxides of iron, primarily the minerals called magnetite and hematite), or as mixtures. Some elements are abundant on Earth. For example, the elements hydrogen and oxygen, as the compound water, H2O, make up the bulk of Earth's oceans, seas, lakes, rivers, and ponds, and make up the bulk (mass) of living cells and multicellular organisms. For another example, the element carbon supplies the backbone of numerous species of essential compounds of all animal and plant life on Earth as well of all the fossil fuels (natural gas, petroleum and coal), which are the remains of plant material that once lived. Some substances may consist of one element only, for instance a nugget of pure gold is made up solely of gold atoms arranged in crystalline form. Very often gold is not pure but an alloy &mdash; a mixture &mdash; of the elements copper, silver, and gold. Oxygen gas consists of entities [see molecule] each having two oxygen atoms chemically bonded to each other, hence the gas consists of the element oxygen only.

Allotropes
Two substances consisting of the same single element may have very different chemical and physical properties. For example, graphite, used as lubricant, and diamond, used to harden drill tips, are both pure carbon. This phenomenon is known as allotropy. Oxygen atoms (O), oxygen gas (O2), and ozone (O3) &mdash; all found in the atmosphere &mdash; are allotropes of the same element, as they have different chemical and physical properties, yet each consists solely of oxygen atoms whose nuclei have identical numbers of protons.

Isotopes
Whereas an element consists of a single species of atom characterized by a unique atomic number, many such species occur in varieties, called isotopes. The isotopes of an element differ among themselves by the number of neutrons in the nucleus, not in the number of protons. As neutrons have mass, and mass similar to that of protons, the isotopes of a given element have differing masses. For example, the most abundant form of hydrogen has a nucleus consisting only of a proton, the fairly rare isotope deuterium has a nucleus that contains one proton and one neutron, and the rarer isotope, tritium, has a nucleus that contains one proton and two neutrons. All three isotopes, while having differing masses, have by definition the same atomic number (=1) and hence are variations, or isotopes, of the same element.

How many chemical elements possible?
There is a maximum to the number of unique elements that can exist since a nucleus contains Z positively charged particles (protons). Those repel each other by Coulomb forces but can remain together by a special nuclear force referred to as the strong nuclear force. At a certain large number of protons the strong nuclear force will begin to lose out to the Coulomb force &mdash; increasingly so with increasing numbers of protons &mdash; and the nucleus will no longer be stable. This is likely to happen between Z = 120 and Z = 130.

By means of a novel mathematical analysis of the properties of the Periodic Table of Elements, Albert Khazan reported in Progress in Physics an upper limit on the atomic number and mass of a chemical element:

 The method of rectangular hyperbolas is developed for the first time, by which a means for estimating the upper bound of the Periodic Table is established in calculating that its last element has an atom mass of 411.663243 and an atomic number (the nuclear charge) of 155.

Transmutation of chemical elements
For a long time, it was thought that elements were unchangeable, that one element could not be converted into another. Alchemists searched for many centuries in vain for the transmutation of the element lead into gold. However, when in 1919 Ernest Rutherford and coworkers showed the transmutation of the element nitrogen into the element oxygen, it became clear that elements can be transmuted.

Aristotle on "elements"
The modern concept of element differs greatly from the Aristotelian concept. Aristotle recognized four elements: fire, water, earth and air, and postulated that they can be converted into each other. He wrote:

Tables

 * See Atomic electron configuration for the orbital occupancies of atoms in their ground state.
 * See also Periodic Table of Elements.

Explanation of names

 * 1) Ag (silver) is from Argentum
 * 2) Au (gold) is from Aurum
 * 3) Cu (copper) is from Cuprum
 * 4) Fe (iron) is from Ferrum
 * 5) Hg (mercury) is from Hydrargyrum
 * 6) K  (potassium) is from Kalium
 * 7) Na (sodium) is from Natrium
 * 8) Pb (lead) is from Plumbum
 * 9) Sb (antimony) is from Stibium
 * 10) Si (silicon) is from Silicium
 * 11) Sn (tin) is from Stannum
 * 12) W  (tungsten) is from Wolfram
 * 13) Man-made elements Z = 112, ..., 118 are not listed