ELEMENTS, CHEMICAL, are entities which have hitherto resisted analysis and which are therefore regarded as simple sub stances. In ancient and mediaeval philosophy, the elements were the four simple substances, earth, air, fire and water, of which all material bodies were supposed to be compounded. In pre-scientific chemistry, the elements were variously enumerated, the usual number being five or six, such as spirit, salt, sulphur, water and earth, or water, oil, air, salt and earth. The modern use of the word dates from the time of Robert Boyle (1661), who described elements as primitive and simple, or perfectly unmixed bodies, which are not made of any other bodies or of one another. From this time onwards the term element was reserved for material substances, and by the end of the 18th century the question of which are true elements had been largely settled. Early chemistry was chiefly concerned with the preparation and description of chemical substances, and no criteria were available to decide whether substances were elements or compounds. About the mid dle of the 18th century, however, chemistry became definitely a quantitative science based on weighing, and the criteria became available. As the weight of a compound is the sum of the weights of its elements, an element is necessarily lighter than any of its compounds. An element is therefore a chemical substance which, by being changed into other chemical substances, invariably in creases in weight. By means of this rule and the experimental chemical evidence relative thereto, A. L. Lavoisier in 1789 was able to compile the first scientific list of elements.
According to the knowledge of his time he regarded the alkalis as elements, although he remarked that they were rather similar to certain oxides, and therefore might possibly contain oxygen; the truth of this was proved at a later date by Humphry Davy. But the inconsistency of the reformer may be observed with the French scientist. He included "heat and light" in his list of ele ments, although he knew that neither of them had weight and that neither filled his definition of an element.
However an element be defined there is always one anomaly, in that there can exist more than one chemical substance containing one and the same kind of atom. On electrolyzing water, the com ponent hydrogen atoms are set free at one electrical pole and the component oxygen atoms at the other pole. The oxygen atoms, however, do not appear as such, but combine with one another in two different ways to form two different chemical sub stances, both of which are gases containing nothing but oxygen atoms. These substances are ordinary oxygen gas, composed of two atoms per molecule, and ozone, composed of three atoms per molecule. There are thus three different things which are all strictly within the definition of the element, viz., single oxygen atoms, diatomic oxygen molecules, and triatomic oxygen mole cules. This anomaly in nomenclature arises from the fact that the element oxygen was identified long before the possibility was considered that elements consisted of atoms capable of various combinations to yield different chemical substances. Usually, how ever, no confusion arises, because in the discussion of chemical elements the atom is understood, while in the discussion of ele mentary substances the polyatomic molecules are implied.
When an element can exist in more than one modification, it is said to exhibit allotropy (q.v.), the different forms or chemical substances being termed allotropes or allotropic modifications of the element. The conception of elements is further complicated by the fact that radioactive chemical substances are known which are absolutely identical in all chemical and physical properties save those of radioactive origin and disintegration. To these sub stances F. Soddy in 1911 gave the name isotopes (see RADIO ACTIVITY). In the same year Sir J. J. Thomson made the impor tant discovery that the element neon (atomic weight 20.2) is a mixture of two atoms differing only in atomic weight, the lighter isotope having the atomic weight 20 and the heavier 22. Since then F. W. Aston, using a modification of the same method of positive ray analysis, has shown that about half of the known elements are mixtures of atoms having whole-number atomic weights (see ISOTOPES) .
The elements may be divided into two classes, metals and non metals. Though the line of demarcation is not sharp, it is usual to assign to the non-metals the eleven gaseous elements, hydrogen, nitrogen, oxygen, fluorine, chlorine, helium, neon, argon, krypton, xenon and radon; the liquid element, bromine; and the nine solid elements, boron, carbon, silicon, phosphorus, arsenic, sulphur, selenium, tellurium and iodine. All the remaining elements are metals, and with the exception of mercury, which is liquid, are solids.
The number of elements is limited. Reference to the periodic classification (q.v.) shows that only 92 elements up to the heav iest, uranium, can possibly exist. Of these, 90 are now known, though three of them, masurium (43), illinium (61) and rhenium (75), are only known with certainty from distinctive lines in their X-ray spectra, their atomic weights having not yet been ascer tained. The two missing elements are a halogen (85) and an alkali metal (87). These numbers, which relate to the order in the classification, and to the numbers of electrons in the atoms, are known as atomic numbers (q.v.). Elements are thus charac terized not only by distinctive physical and chemical properties but by numbers indicating that they differ in structure by one electron in successive atoms in the periodic classification.
Only about 3o elements occur in nature in the free state on the earth (see Table II.), though it is probable that in the sun and the hotter stars all elements are present in the uncombined condi tion. The commonest of these earth elements, namely, oxygen, comprises free and in combination about half the weight of the earth's crust, while the rarest, radon, probably amounts to less than a million millionth of the earth's atmosphere. Table I. gives the average composition of terrestrial material half a mile deep, including the earth's crust, oceans and atmosphere.
From this table it is evident that the four light elements, oxygen, hydrogen, silicon and aluminium, comprise together about 90% of the earth's crust both by weight and by proportion of atoms, and that a dozen light elements together account for 98% of terrestrial matter.
The general properties of the elements, even of those very close in atomic weight, are extremely diverse. Carbon, for example, has a boiling point about 3,700° C, while the next heavier element, nitrogen, has a boiling point nearly 4,000° lower at —195° C, only 78° C above the absolute zero temperature of —273° C. The melt ing and boiling points of an element are in some cases quite close together, and in others very far apart. The boiling point of helium, for example, is only about 2° above its melting point, —271° C, the lowest temperature that has ever been attained by artificial means, whereas tungsten melts about 3,000° C and boils about 6,000° C, the highest melting and boiling points known for any element. Table II. gives the melting and boiling points, where known, for all the elements.
The outstanding chemical constants of the elements are their atomic weight, atomic number and valency. Atomic weights, re ferred to the unit of one-sixteenth of that of oxygen (taken as i6), range from i.008 in the case of the lightest element, hydro gen, to 238.17 for the heaviest element, uranium (see ATOMIC WEIGHTS). Atomic weights have not yet been determined for masurium (43), illinium (61), rhenium (75), polonium (84), actinium (89) and protoactinium (91). The atomic numbers 43, 61 and 75 are known from X-ray spectra, and 84, 89 and 91 are inferred from the radioactive disintegration series. The atomic weight, 222, of the radioactive noble gas, radon (86), is not com pletely certain, though, by analogy with the other noble gases, it is assumed to be double the known density.
The elements may be classified into two species by the valency criterion, first, those that have no valency (the six noble gases typified by helium), and second, all the elements that possess valency (the remaining 86 elements). The valency of an element is always an integer, as follows from the law of simple multiple proportions by weight, and, as indicated by modern theory, is the number of its electrons that are effective in chemical combination. The highest known valency is eight, exemplified by the octa fluoride of osmium and the tetroxides of ruthenium and osmium. It is also known from the radioactive disintegration series that the noble gas, radon, has eight electrons that are potential valency factors. According to valency, the elements fall into eight families, the eight groups of the periodic classification with valency from one to eight. For the majority of the elements, valency is not a fixed number, but varies for any one element within certain limits. This variation occurs in two different ways, some elements ex hibiting variation always of two units from the maximum to a minimum, whereas others exhibit variation by a single unit: Reference to the periodic classification (q.v.) shows that this variation is characteristic of certain types of elements, the ele ments of the typical classification exhibiting variation by two units of valency, whereas the remaining 48 elements (known as transition elements) exhibit variation by one unit. All the transi tion elements are metals, and, with the exceptions of zinc, cad mium and mercury, have boiling points above 5,900° C. The tran sition elements, moreover, are the only known elements which give rise to coloured salts in their highest condition of valency, and most of them form coloured salts at every condition of valency. The valency variation of two units for the elements of the typical classification is interpreted as indicating that each atom contains one pair or two pairs of valency electrons, and that the electrons forming a pair are equally firmly bound to the atom. The valency variation of one unit for the transition elements in dicates that this system of electron pairs does not exist except where the minimum valency is two. With the exceptions of cop per, silver and gold, every transition element has a minimum valency of two, and thus possesses a single pair of electrons that are equally and least firmly bound to the atom. To these two types of valency electronic structure are to be ascribed not only the general and characteristic chemical properties of the elements, but also the specialized chemical properties, which differentiate the typical from the transition series of elements. For transmuta tion of elements see article under that title.