Chemical elements

Chemical elements, in one conceptual sense of the term, 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., protons) 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. Familiar species of atoms include oxygen, copper, gold, and mercury — among the 94 naturally occurring species of atoms on Earth. See tables below.

In the second commonly used conceptual sense of the term, chemical elements refers to chemical pure substances each composed of a collection of atoms solely of a single species, or type, of atoms, 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 are segments of wire made solely of copper atoms, and rolls of aluminum foil made solely of aluminum atoms. This second sense, or concept, makes the term chemical element somewhat more tangible, defining it in terms of a substance. 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. Moreover the modern definitions of chemical elements described in the introduction above do not accord with the implication of the older conception of a chemical element as a substance having identical component parts, 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 −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 laptops and cellphones; 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; chorine (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; 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 jewlery, coins, 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 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 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.

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) — 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.

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 — 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.

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:

"….the elements are the primary constituents of bodies.... An element, we take it, is a body into which other bodies may be analysed, present in them potentially or in actuality (which of these, is still disputable), and not itself divisible into bodies different in form. That, or something like it, is what all men in every case mean by element….every body is either an element or composed of elements…."     —Aristotle. On the Heavens Book III. Translated by J.L. Stocks.

Tables

See Atomic electron configuration for the orbital occupancies of atoms in their ground state.

See also Periodic Table of Elements.

Chemical elements sorted on chemical symbol (CS)

CS	Name	Z		CS	Name	Z		CS	Name	Z
Ac	Actinium	89		Gd	Gadolinium	64		Po	Polonium	84
Ag	Silver	47		Ge	Germanium	32		Pr	Praseodymium	59
Al	Aluminum	13		H	Hydrogen	1		Pt	Platinum	78
Am	Americium	95		He	Helium	2		Pu	Plutonium	94
Ar	Argon	18		Hf	Hafnium	72		Ra	Radium	88
As	Arsenic	33		Hg	Mercury	80		Rb	Rubidium	37
At	Astatine	85		Ho	Holmium	67		Re	Rhenium	75
Au	Gold	79		Hs	Hassium	108		Rf	Rutherfordium	104
B	Boron	5		I	Iodine	53		Rg	Roentgenium	111
Ba	Barium	56		In	Indium	49		Rh	Rhodium	45
Be	Beryllium	4		Ir	Iridium	77		Rn	Radon	86
Bh	Bohrium	107		K	Potassium	19		Ru	Ruthenium	44
Bi	Bismuth	83		Kr	Krypton	36		S	Sulfur	16
Bk	Berkelium	97		La	Lanthanum	57		Sb	Antimony	51
Br	Bromine	35		Li	Lithium	3		Sc	Scandium	21
C	Carbon	6		Lr	Lawrencium	103		Se	Selenium	34
Ca	Calcium	20		Lu	Lutetium	71		Sg	Seaborgium	106
Cd	Cadmium	48		Md	Mendelevium	101		Si	Silicon	14
Ce	Cerium	58		Mg	Magnesium	12		Sm	Samarium	62
Cf	Californium	98		Mn	Manganese	25		Sn	Tin	50
Cl	Chlorine	17		Mo	Molybdenum	42		Sr	Strontium	38
Cm	Curium	96		Mt	Meitnerium	109		Ta	Tantalum	73
Co	Cobalt	27		N	Nitrogen	7		Tb	Terbium	65
Cr	Chromium	24		Na	Sodium	11		Tc	Technetium	43
Cs	Cesium	55		Nb	Niobium	41		Te	Tellurium	52
Cu	Copper	29		Nd	Neodymium	60		Th	Thorium	90
Db	Dubnium	105		Ne	Neon	10		Ti	Titanium	22
Ds	Darmstadtium	110		Ni	Nickel	28		Tl	Thallium	81
Dy	Dysprosium	66		No	Nobelium	102		Tm	Thulium	69
Er	Erbium	68		Np	Neptunium	93		U	Uranium	92
Es	Einsteinium	99		O	Oxygen	8		V	Vanadium	23
Eu	Europium	63		Os	Osmium	76		W	Tungsten	74
F	Fluorine	9		P	Phosphorus	15		Xe	Xenon	54
Fe	Iron	26		Pa	Protactinium	91		Y	Yttrium	39
Fm	Fermium	100		Pb	Lead	82		Yb	Ytterbium	70
Fr	Francium	87		Pd	Palladium	46		Zn	Zinc	30
Ga	Gallium	31		Pm	Promethium	61		Zr	Zirconium	40

Chemical elements sorted on atomic number (Z)

Z	Name	CS		Z	Name	CS		Z	Name	CS
1	Hydrogen	H		38	Strontium	Sr		75	Rhenium	Re
2	Helium	He		39	Yttrium	Y		76	Osmium	Os
3	Lithium	Li		40	Zirconium	Zr		77	Iridium	Ir
4	Beryllium	Be		41	Niobium	Nb		78	Platinum	Pt
5	Boron	B		42	Molybdenum	Mo		79	Gold	Au
6	Carbon	C		43	Technetium	Tc		80	Mercury	Hg
7	Nitrogen	N		44	Ruthenium	Ru		81	Thallium	Tl
8	Oxygen	O		45	Rhodium	Rh		82	Lead	Pb
9	Fluorine	F		46	Palladium	Pd		83	Bismuth	Bi
10	Neon	Ne		47	Silver	Ag		84	Polonium	Po
11	Sodium	Na		48	Cadmium	Cd		85	Astatine	At
12	Magnesium	Mg		49	Indium	In		86	Radon	Rn
13	Aluminum	Al		50	Tin	Sn		87	Francium	Fr
14	Silicon	Si		51	Antimony	Sb		88	Radium	Ra
15	Phosphorus	P		52	Tellurium	Te		89	Actinium	Ac
16	Sulfur	S		53	Iodine	I		90	Thorium	Th
17	Chlorine	Cl		54	Xenon	Xe		91	Protactinium	Pa
18	Argon	Ar		55	Cesium	Cs		92	Uranium	U
19	Potassium	K		56	Barium	Ba		93	Neptunium	Np
20	Calcium	Ca		57	Lanthanum	La		94	Plutonium	Pu
21	Scandium	Sc		58	Cerium	Ce		95	Americium	Am
22	Titanium	Ti		59	Praseodymium	Pr		96	Curium	Cm
23	Vanadium	V		60	Neodymium	Nd		97	Berkelium	Bk
24	Chromium	Cr		61	Promethium	Pm		98	Californium	Cf
25	Manganese	Mn		62	Samarium	Sm		99	Einsteinium	Es
26	Iron	Fe		63	Europium	Eu		100	Fermium	Fm
27	Cobalt	Co		64	Gadolinium	Gd		101	Mendelevium	Md
28	Nickel	Ni		65	Terbium	Tb		102	Nobelium	No
29	Copper	Cu		66	Dysprosium	Dy		103	Lawrencium	Lr
30	Zinc	Zn		67	Holmium	Ho		104	Rutherfordium	Rf
31	Gallium	Ga		68	Erbium	Er		105	Dubnium	Db
32	Germanium	Ge		69	Thulium	Tm		106	Seaborgium	Sg
33	Arsenic	As		70	Ytterbium	Yb		107	Bohrium	Bh
34	Selenium	Se		71	Lutetium	Lu		108	Hassium	Hs
35	Bromine	Br		72	Hafnium	Hf		109	Meitnerium	Mt
36	Krypton	Kr		73	Tantalum	Ta		110	Darmstadtium	Ds
37	Rubidium	Rb		74	Tungsten	W		111	Roentgenium	Rg

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

Glossary

• matter: In chemistry, 'matter' refers to anything that takes up space and has mass. This common definition used by chemists 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.

• substance: