User:Anthony.Sebastian/SebastianSandbox/Chemical elements

Introduction
Chemical elements are those particular types of matter that are composed of a population solely of a single type (or species) of atom, as exemplified by a segment of wire made solely of copper atoms, say, or a roll of kitchen foil made solely of aluminum atoms. Some chemists refer to chemical elements so defined as 'elementary substances'. The number of Earth´s naturally occurring types of atoms, established as 94, determines the number of Earth´s naturally occurring types of chemical elements, of which copper and aluminum are just two examples. The distinguishing characteristic of each of the 94 naturally occurring types of atoms is the number of protons in the nucleus of its atoms, referred to as the atomic number, Z, unique for each type of atom and for each of the corresponding chemical element.

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 names of the elements are of historical origin and may differ among languages for an element. 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.

In addition to the 94 elements that occur naturally on Earth, about 23 other known elements that do not occur naturally on Earth have been man-made and are characterized by their constituent atoms having very short life times and being radioactive. The exclusively man-made elements on Earth run from Z = 95 to 118.

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 cellphones, oxygen (O), in the air we breathe, neon (Ne), in 'neon' lights, sodium (Na), which is present in table salt that nutritionists advise using sparingly in foods and aluminum (Al), used as foil for wrapping leftovers.

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 populations of atoms, often with the atoms of other elements, as compounds (e.g., iron ore, a population 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 oganisms. 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 compounds 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.

Some of the 94 elements, such as the gas neon, are very rare on Earth. Some elements are stable, and will live as long as the universe, while some, known as the radioactive elements,  have finite life times and decay into other elements while emitting radiation. For example, plutonium is a well-known radioactive element.

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 due to the fact that 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.

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:

Glossary

 * matter: anything that takes up space and has mass. This is the common definition used by chemists, and suffices to enable explaining the concept of 'chemical element' at one level of explanation. Physicists define 'matter' in terms of energy and force fields. See matter.