|Welcome to the Periodic Kingdom...This is the kingdom of the chemical elements, the substances from which everything tangible is made. It is not an extensive country, for it consists of only a hundred or so regions (as we shall often term the elements), yet it accounts for everything material in our actual world. From the hundred elements that are at the center of our story, all planets, rocks, vegetation, and animals are made. These elements are the basis of the air, the oceans, and the Earth itself. We stand on the elements, we eat the elements, we are the elements. Because our brains are made up of elements, even our opinions are, in a sense, properties of the elements and hence inhabitants of the kingdom...The kingdom is not an amorphous jumble of regions, but a closely organized state in which the character of one region is close to that of its neighbor. There are few sharp boundaries. Rather, the landscape is largely characterized by transitions: savannah blends into gentle valleys, which gradually deepen into almost fathomless gorges; hills gradually rise from plains to become towering mountains. |
The term "chemical element", in one conceptual sense, refers to a species, or type, of atom, of which nature has produced some 90+ types, each with countless numbers of token atoms.  The distinguishing characteristic of a species of atoms is the number of units of positive charge in the nucleus of its atoms, i.e., the number of protons, referred to as the atomic number of the element, each proton carrying one unit of positive charge, the 'atomic number' symbolized, Z. Thus, the atomic number is unique for each species of atom and therefore for each corresponding chemical element. The chemical elements, oxygen (Z=8), copper (Z=29), gold (Z=79), and mercury (Z=80), and all the other chemical elements, thus each have a unique atomic number, specified in tables below.
In summary, chemists conceive of oxygen and the other chemical elements as species of atoms, defined by their atomic numbers. A "periodic" table of elements, with rows and columns containing elements which exhibit similar properties, can be found here.
In a second common conceptual sense, "chemical elements" refers to an amassment of matter, typically a so-called macroscopic amassment, composed of a collection of many atoms of a single species, or type, of atom, again each species 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. Moreover, the term 'substance' generally implies a 'pure' substance, one composed entirely of identical unit components, whereas, as will be discussed below, many species of atoms, each with their unique atomic number, exist in their natural state as collections, or mixtures, of atoms with identical numbers of protons but differing numbers of neutral particles, called neutrons, in their atomic nuclei — the varieties of a species referred to as 'isotopes' of the chemical element (vide infra). That proviso regarding the implied 'pure' in the use of 'substance' should be remembered when using the term 'elementary substance'.
Familiar examples of elementary substances include segments of electrical current-conducting wire made solely of copper atoms, and handy rolls of flexible kitchen foil made solely of aluminium 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 collection of atoms of a single species, implying perceptual tangibility.
Though chemical elements can be conceptualized as elementary 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. Aluminium 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 body of matter that ordinary chemical methods could not segregate into separate distinct simpler bodies of matter. Some introductory chemistry textbooks still give that older definition as primary, as does the current (2009) edition of the Encyclopedia Britannica Online. Since early chemists could not have known whether future technology would provide a chemical method to further simplify a presumed chemical element, that older concept was inherently incoherent.
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 body of matter having identical component parts (simple bodies), as methods have been developed, even 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. In current (2010) atomic "models", it is believed that Z electrons (of charge −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 Plutonium (Z > 94) have not been found naturally occurring on Earth, and so all samples of these are man-made and radioactive, though some are created in supernova explosions for very brief periods. Non-radioactive elements are stable, and appear to "live" (remain unchanged) as long as the universe, while the radioactive elements, have finite life times (defined by their "half-lives") and decay into other elements while emitting radiation. The so-called "transuranic elements" run from Z = 93 to 118. The elements with Z = 1, …, 91 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; aluminium (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 — whether solid, liquid or gas — 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 — a mixture — 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.
Two elementary substances consisting of the same single type of atom (chemical 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)—all found in the atmosphere—are allotropes of the same element, oxygen, as they have different chemical and physical properties, yet each consists solely of oxygen atoms whose nuclei have identical numbers of protons.
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 virtue of the nuclear force, more fundamentally referred to as the strong force. At a certain large number of protons the strong nuclear force will begin to lose out to the Coulomb force—increasingly so with increasing numbers of protons—and the nucleus will no longer be stable. This is likely to happen between beyond Z = 150.
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.
It is expected that a nucleus with Z = 120 and N = 184 (number of neutrons) will be the center of an island of enhanced stability.
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 co-workers 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:
"….the elements are the primary constituents of bodies....
- See Atomic electron configuration for the orbital occupancies of atoms in their ground state.
- See also Periodic Table of Elements.
Explanation of names
- Ag (silver) is from Argentum
- Au (gold) is from Aurum
- Cu (copper) is from Cuprum
- Fe (iron) is from Ferrum
- Hg (mercury) is from Hydrargyrum
- K (potassium) is from Kalium
- Na (sodium) is from Natrium
- Pb (lead) is from Plumbum
- Sb (antimony) is from Stibium
- Si (silicon) is from Silicium
- Sn (tin) is from Stannum
- W (tungsten) is from Wolfram
- Man-made elements Z = 112, ..., 118 are not listed
New elements added
- "For the first time in four years, the International Union of Pure and Applied Chemistry (IUPAC) has approved the introduction of new elements. The periodic table of the chemical elements is growing by four elements. These are the elements with atomic numbers 113, 115, 117, and 118. They complete the seventh row of the periodic table.
- "The discoverers from Japan, Russia, and the USA will now be invited to suggest permanent names and symbols. So far, the elements have temporary working names and symbols: Ununtrium (113, symbol Uut), Ununpentium (115, Uup), Ununseptium (117, Uus) and Ununoctium (118, Uuo). Element 113 will be the first element to be named in Asia – its discoverers are sitting at Riken Institute in Japan".
- Atkins PW. (1995) The Periodic Kingdom: A Journey into the Land of the Chemical Elements. New York: Basic Books.
- International Union of Pure and Applied Chemistry (IUPAC) definition of chemical element. From the website of the IUPAC's Goldbook published as the Compendium of Chemical Terminology.
- Note: The type/token distinction can be exemplified by considering the different 'types' of coins (penny, nickel, dime, quarter, half-dollar, silver-dollar), each type of coin having many individual coins of that type, referred to as 'tokens' of that type of coin. If you have 50 dimes, you have 50 tokens of the 'dime' type of coin. The 'potassium' type of chemical element comprises countless numbers of individual atoms, or 'tokens' of the potassium type. Types of chemical element may also be referred to as a 'species', each with countless individual members, the tokens of the species.
- Note: 'Substance' remains undefined by the International Union of Pure and Applied Chemistry (IUPAC), but chemists typically define it in terms of matter, or imply matter, not as physicists view matter, but in its sense as something that takes up space and has mass. In particular they refer to substances having a uniform composition, and often restricting them to 'pure substances', including as such elementary substances, though the latter may consist of mixtures of isotopes, and compounds, but not to mixtures of elementary substances, such as alloys. See accompanying reference by PW Atkins (1989)
- Atkins PW. (1989) General Chemistry. Scientific American Books. ISBN 0716719401.
- Scott T., Eagleson M. (1994) Concise Encyclopedia of Chemistry. Walter de Gruyter. ISBN 9783110114515. Google Book
- Aluminum: Chemical of the Week Chemistry Department. University of Wisconsin.
- Kragh H. (2000) Conceptual Changes in Chemistry: The Notion of a Chemical Element, ca. 1900-1925. Stud. Hist. Phil. Mod. Phys. 31(4):435-450.
- Enghag P. (2004) Encyclopedia of the Elements. WILEY-VCH Verlag GmbH & Co KGaA. ISBN 3-527-30666-8.
- Roundy WH Jr. (1989) What is an Element? J. Chem. Educ. 66:729-730.
- Holden NE. (2001) History of the Origin of the Chemical Elements and Their Discoverers. Prepared for the 41st IUPAC General Assembly in Brisbane, Australia, June 29th - July 8th, 2001.
- Note: A typical living cell consists of 75-85% water by mass.
- Allotrope Names of the Elements. | List of elements sortable alphabetically or by atomic number. For carbon, the source lists the naturally-occurring and man-made allotropes: Graphite, Diamond, Amorphous carbon, Lonsdaleite, Fullerene, Carbon nanotube.
- Khazan A. (2007) Upper Limit in the Periodic Table of Elements. Progress in Physics 1:38-41. | Download article PDF
- Physics Today, April 2010, p. 12
- –Chemistry Views, New Elements added, 113, 115, 117, 118.