Chemical elements
Chemical elements — "the substances from which everything tangible is made"[1] — are those particular types of matter of which a macroscopic sample[2] of each element type is constituted of a population solely of a single type (or species) of atom — each element type unique in virtue of the number of protons in each of its sample population's constituent atoms' nuclei, referred to as the atomic number of the element type. On Earth 94 different types of atoms occur in nature, and therefore nature supplies 94 different element types, each element type having its unique atomic number, and each characterized by a unique set of physical and chemical properties.[3]
People from all walks of life know something about many different elements from everyday life, even if they do not recognize them as elements. They include: Hydrogen (H1), used to make hydrogen bombs. Helium (He2), used to make party balloons float. Lithium (Li3), used to make batteries for cellphones. Oxygen (O8), in the air we breathe. Neon (Ne10), in 'neon' lights. Sodium (Na23, nutritionists advise consuming sparingly in foods and from salt shaker.
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 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) 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.[4] 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 — 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 substances consisting of the same single element may have very different chemical and physical properties, e.g., graphite, used as lubricant, and diamond, used to harden drill tips, 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, as they have different chemical and physical properties, yet each consists solely of [oxygen] atoms whose nuclei have identical numbers of protons,
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.
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.
As noted, the atoms of each of the elements are distinguished by a unique atomic number, an integral (whole) number, symbolized Z, indicating the number of protons in the atom's nucleus. 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 −e, or negative e, and of much smaller mass than the proton) "orbit" the nucleus of an atom, so that an atom as a whole is electrically neutral. The following elements have their values of Z (Z-values) in brackets: hydrogen(1), oxygen(8), carbon(6), neon(10), plutonium(94). The naturally occurring elements have Z-values from 1 to 94 (with plutonium being extremely rare in nature and mainly man-made). The exclusively man-made elements on Earth run from Z = 95 to 118. 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 a unique and universal label of an element, as is its international chemical symbol consisting of one or two letters.
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.
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 — increasingly so with increasing numbers of protons — and the nucleus will no longer be stable. This is likely to happen between Z = 120 and Z = 130.
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.
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.... |
Tables
- See Atomic electron configuration for the orbital occupancies of atoms in their so-called 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
References and notes
- ↑ Atkins PW. (1995, 1997) The Periodic Kingdom: A Journey into the Land of the Chemical Elements. (Full-Text). New York: Basic Books. ISBN 9780465072668.
- Publisher´s Description: Come on a journey into the heart of matter—and enjoy the process!—as a brilliant scientist and entertaining tour guide takes you on a fascinating voyage through the Periodic Kingdom, the world of the elements. The periodic table, your map for this trip, is the most important concept in chemistry. It hangs in classrooms and labs throughout the world, providing support for students, suggesting new avenues of research for professionals, succinctly organizing the whole of chemistry. The one hundred or so elements listed in the table make up everything in the universe, from microscopic organisms to distant planets. Just how does the periodic table help us make sense of the world around us? Using vivid imagery, ingenious analogies, and liberal doses of humor P. W. Atkins answers this question. He shows us that the Periodic Kingdom is a systematic place. Detailing the geography, history and governing institutions of this imaginary landscape, he demonstrates how physical similarities can point to deeper affinities, and how the location of an element can be used to predict its properties. Here’s an opportunity to discover a rich kingdom of the imagination kingdom of which our own world is a manifestation.
- ↑ Note: A sample large enough to be observed without a microscope.
- ↑ According to James B. Calvert: The trite phrase "the 92 naturally-occurring chemical elements" is often seen, but is incorrect. There are only 88 naturally-occuring chemical elements. The elements 43 [technetium], 61 [promethium], 85 [astatine] and 87 [francium] have no stable isotopes, and none of long half-life, so they are not naturally present. Small amounts are made in nuclear reactions induced by cosmic rays and nuclear tests, but these soon disappear. If you protest that these should be included, then so should Np [neptunium] and Pu [plutonium], which are produced by the absorption of neutrons arising from spontaneous fission of uranium and thorium, and then there would be 94 naturally-occurring elements. If you wait long enough, there will only be 81 naturally-occurring elements, since everything beyond lead has only unstable isotopes, though some are of very long half-life, and have survived since the beginning, fathering their radioactive series. Any way you look at it, there are not just 92 naturally-occurring chemical elements. See: http://mysite.du.edu/~jcalvert/phys/92.htm
- ↑ Note: A typical living cell consists of 75-85% water by mass.