Encyclopedia
A
chemical element, often called simply an
element, is a
substance that cannot be decomposed or transformed into other chemical substances by ordinary
chemical processes. All matter fundamentally consists of these elements and as of 2006, 118 unique elements have been discovered or artificially created. The smallest particle of such an element is an
atom, which consists of
electrons centered about a
nucleus of
protons and
neutrons.
Chemistry terminology
Earlier an
element or
pure element was defined as a substance which "can't be further broken down into another compound with different chemical properties" -- which should be taken to mean it consists of atoms of one element. However, due to
allotropy, the
isotope effect, and the confusion with the more useful term referring to the general class of atoms , this usage is in disfavor amongst contemporary chemists, and sees restricted, mostly historical, use. This definition was motivated by the observation that these elements could not be dissociated by chemical means into other compounds. For example, water could be converted into hydrogen and
oxygen, but hydrogen and oxygen could not be further decomposed, thus "elemental". There are also many counterexamples . This article will concern itself with the latter definition.
Description
The lightest elements are
hydrogen and
helium. All the heavier elements are made, both naturally and artificially, through various methods of
nucleosynthesis.
As of 2006, there are 118 known elements: 93 occur naturally on earth , and 94 have been detected in the universe at large. The 23 elements not found on earth are derived artificially;
technetium was the first purportedly non-naturally occurring element to be synthesized, in 1937, although trace amounts of technetium have since been found in nature. All artificially derived elements are radioactive with short half-lives, so if any atoms of these elements were present at the formation of Earth are extremely likely to have already decayed.
Lists of the elements by name, by symbol, by atomic number,
by density, by melting point, and by boiling point as well as Ionization energies of the elements are available. The most convenient presentation of the elements is in the periodic table, which groups elements with similar chemical properties together.
Atomic number
The atomic number of an element,
Z, is equal to the number of protons which defines the element. For example, all
carbon atoms contain 6 protons in their
nucleus, so for carbon Z=6. These atoms may have different amounts of neutrons, and are known as isotopes of the element. The atomic mass of an element,
A, is measured in unified atomic mass units is the average mass of all the atoms of the element in an environment of interest . Since electrons are of negligible mass, and neutrons are barely more than the mass of the proton, this usually corresponds to the sum of the protons and neutrons in the nucleus of the most abundant isotope, though this is not always the case .
Atomic mass
The atomic masses that are given on the periodic table are calculated by the following method. As an example, assume there exists three isotopes of element X and their respective atomic masses are 10, 20 and 30 AMU for sake of demonstration. Now also assume that 50% of the isotopes of element X are the 10 AMU version and the two heavier isotopes each account for 25% of the total number of atoms of this hypothetical element. As a result 10 * 0.5 = 5 AMU and 20 * 0.25= 5 AMU and 30 * 0.25 = 7.5 AMU. The average atomic mass that results is 17.5 AMU. The reason is because the method to calculate the average mass takes into account the relative abundance of all of the isotopes of an element, which is multiplied against their individual masses.
Isotopes
Some isotopes are
radioactive and decay into other elements upon radiating an alpha or beta particle. Some elements have no nonradioactive isotopes, in particular all elements with atomic numbers greater than 83.
Nomenclature
The naming of elements precedes the atomic theory of matter, although at the time it was not known which chemicals were elements and which compounds. When it was learned, existing names
In the second half of the twentieth century physics laboratories became able to produce nuclei of chemical elements that have a half life too short for them to remain in any appreciable amounts. These are also named by IUPAC, which generally adopts the name chosen by the discoverer. This can lead to the controversial question of which research group actually discovered an element, a question which delayed the naming of elements with atomic number of 104 and higher for a considerable time. .
Precursors of such controversies involved the nationalistic namings of elements in the late nineteenth century. For example
lutetium was named in reference to Paris, France, the Germans were reluctant to relinquish naming rights to the French, often calling it
cassiopeium. The British discoverer of
niobium originally named it
columbium, in reference to the
New World. It was used extensively as such by American publications prior to international standardization.
Chemical symbols
- For the listing of current and not used Chemical symbols, and other symbols that look like chemical symbols, please see List of elements by symbol.
Specific chemical elements
Before chemistry became a science,
alchemists had designed arcane symbols for both metals and common compounds. These were however used as abbreviations in diagrams or procedures; there was no concept of one atoms combining to form molecules. With his advances in the atomic theory of matter,
John Dalton devised his own simpler symbols, based on circles, which were to be used to depict molecules.
The current system of chemical notation was invented by
Berzelius. In this typographical system chemical symbols are not used as mere abbreviations - though each consists of letters of the
Latin alphabet - they are symbols intended to be used by peoples of all languages and alphabets. The first of these symbols were intended to be fully universal; since Latin was the common language of science at that time, they were abbreviations based on the
Latin names of metals - Fe comes from Ferrum, Ag from Argentum. The symbols were not followed by a period as abbreviations were. Later chemical elements were also assigned unique chemical symbols, based on the name of the element, but not necessarily in English. For example,
sodium has the chemical symbol 'Na' after the Latin
natrium. The same applies to "W" for tungsten, "Hg" for mercury, "K" for
potassium, and "Sb" for
antimony.
Chemical symbols are understood internationally when element names might need to be translated. There are sometimes differences; for example, the Germans have used "J" instead of "I" for iodine, so the character would not be confused with a
roman numeral.
The first letter of a chemical symbol is always capitalized, as in the preceding examples, and the subsequent letters, if any, are always lower case .
General chemical symbols
There are also symbols for series of chemical elements, for comparative formulas. These are one capital letter in length, and the letters are reserved so they are not permitted to be given for the names of specific elements. For example, an "X" is used to indicate a variable group amongst a class of compounds , while "R" is used for a radical, meaning a compound structure such as a hydrocarbon chain. The letter "Q" is reserved for "heat" in a chemical reaction. "Y" is also often used as a general chemical symbol, although it is also the symbol of Yttrium. "Z" is also frequently used as a general variable group. "L" is used to represent a general ligand in inorganic and organometallic chemistry. "M" is also often used in place of a general metal.
Isotope symbols
Although not officially used, in nuclear physics the three main isotopes of the element
hydrogen are often written as H for protium, D for
deuterium and T for
tritium. This is in order to make it easier to use them in chemical equations, as it replaces the need write out the AMU for each isotope. It is written like this:
D
2O
Instead of writing it like this:
2H
2O
See also
- Abundance of the chemical elements
- Compound
- Chemical elements named after people
- Chemical elements named after places
- Chemical symbol
- Chemistry
- Discovery of the chemical elements
- Elements song
- Fictional element
- Periodic table
- Systematic element name
- List of elements by atomic number
External links
- word history and language dictionary
Chemical information
