FORMULAE AND NOMENCLATURE A chemical formula is a collection of letters and numerals indi cating the elementary composition of a compound substance. The symbol for an atom of an element is either the capital letter of its English or Latin name, as C for carbon and K for potassium (Latin, kaliur), or the capital letter followed by a small letter, as Co for cobalt and Cr for chromium. The qualitative composi tion of a compound substance is indicated by a formula consisting of the symbols of the elements in juxtaposition, as CO for carbon oxide. Chemical formulae, however, convey quantitative as well as qualitative composition. Empirical formulae indicate the com ponent elements and the simplest ratio between them, as for iron oxide, the subscript numerals signifying a ratio of two atoms of iron to three of oxygen. Rational formulae indicate further the actual number of atoms of each element in one com pound molecule, i.e. in the smallest portion of the compound that can exist as a separate entity. The empirical formula of acetylene, for example, is CH, but the rational formula is known to be Empirical and rational formulae are, however, often identi cal. A chemical formula generally attempts to show the scheme of combination between the various atoms comprising a molecule. For example, the rational formula of thorium sulphate is but is usually rearranged as the subscript numeral 2 indicating that two or sulphate groupings are separate radi cals independently in combination with one atom of thorium. Rational formulae thus rearranged as structural formulae are in general use, particularly in organic chemistry. and conventional schemes have been adopted to convey the exact mode of combina tion in compounds with many different identifiable radicals.
By common agreement, the names of compounds of atoms of two elements are formed from the name of the electro-positive element followed by the name of the electro-negative element inflected by the suffix ide, as in sodium chloride, calcium oxide, zinc sulphide. When two elements form more than one com pound, Greek numerical prefixes are generally used to inflect the electro-negative term, as in carbon monoxide, carbon dioxide, phosphorus trioxide and phosphorus pentoxide. The Latin pre fix sesqui is, however, used to indicate one and a half, as in the case of chromium sesquioxide in order to distinguish it from chromium trioxide Elements, which form more than one oxide and give rise to corresponding series of salts, have the different series distinguished by an inflecting suffix to the electro-positive term, the numerical prefix to the electro-negative term being omitted. The suffix ous is used to indicate the lower and the suffix ic to indicate the higher condition of oxidation, as in the copper compounds, cuprous oxide and cupric oxide (Cu0).
Oxides may be classified as acidic or basic, according to whether they neutralize bases or acids. Acidic oxides on solution in water give rise to acids, sulphur dioxide for example, combining with water to form sulphurous acid The more highly oxygenated sulphur trioxide yields sulphuric acid The terminations ous and ic are generally used to distinguish the acids derived from the lower and higher acidic oxide. Basic oxides also usually combine with water, but form bases not acids, basic barium oxide (Ba0), for example, combining with water to form the base, barium hydroxide When acids and bases neutralize one another water is eliminated and the same salt is formed as by the combination of the basic and acidic oxide. Barium hydroxide, for example, combines with sulphurous acid to form the salt, barium sulphite the product of the combination of barium oxide and sulphur dioxide. Similarly. barium hydroxide and sulphuric acid yield barium sulphate (BaSO4), the product of the combination of barium oxide with sulphur trioxide. The terminations ite and ate are used to indicate the salts derived from acids terminating in ous and ic, whereas the termination ide is used for salts and compounds in general formed from only two elements, as in the case of oxide, chloride and sulphide. If more than two series of acidic oxides, acids and salts are derived from an element, the inflecting prefix hypo i, used for the series lowest in oxidation, and per for the series highest in oxidation, as in the case of hypochlorous, chlorous, chloric and perchloric acids, and hypochlorites, chlorites, chlorates and perchlorates. As acidic oxides are derived from acids by abstraction of water, such oxides are alternatively referred to as anhydrides, particularly in organic chemistry.
A few oxides and hydroxides are at once basic and acidic, corn bining with either acids or bases, and are termed amphoteric. Aluminium oxide, for example, is amphoteric in yielding alumin ium salts with acids and aluminates with bases, as in the case of aluminium nitrate and sodium aluminate.
The oxygen of acids is replaceable in whole or in part by other elements allied to oxygen, for example sulphur. If one oxygen atom in a sulphate, for example, be replaced by a sulphur atom, the product is a thiosulphate (t/ieion, sulphur), as in the case of sodium thiosulphate or Similarly the replacement of the oxygen by sulphur in potassium carbonate gives potassium thiocarbonate The capacity of an element for combination is not exhausted by simple salt formation by reaction of its oxide or hydroxide' with acids. Aluminium sulphate, for example, is the salt formed by the reaction of aluminium hydroxide with sul phuric acid, but this salt combines, molecule for molecule, with potassium sulphate and water to form a double salt, alum, which has many analogues (see ALUM). Diverse double salts are furnished by other metals in combination with acidic radicals other than sulphates. Magnesium chloride, for example, combines with potassium chloride and water to form the double chloride, carnallite The combina tion between the simple salts in double salts is, however, relatively loose, and the salts are split into their components often merely by solution in water, and the various metallic and acidic radicals are readily detected by the usual analytical tests. In other types of similar compounds, however, the union between the component simple salts is very much closer, and frequently the compounds are not readily decomposed into their components by the usual reagents. Such compounds are referred to as complex salts, and in these cases two or more of the component radicals, acidic or metallic, cannot be detected by the usual analytical processes. The compound of sodium chloride and platinum chloride, sodium platinichloride or chloroplatinate for example, is soluble in water without decomposition and yields none of the ordinary analytical reactions of either chlorides or platinum salts. The complex salt, potassium ferrocyanide or similarly gives none of the usual analytical reactions of cyanides or iron salts.
A new viewpoint of the constitution of these and many other complex compounds was put forward by Alfred Werner in 1891 and 1893 by the promulgation of his theory of co-ordination (q.v.), in which the valency of an atom was regarded as distrib uted in space around the atom and thus partly available for bind ing atoms other than those primarily engaged in the principal valency combination (see AMINES).
The simplest inorganic compounds are the hydrides, compounds of the elements with hydrogen, but until recent years compara tively few hydrides had been prepared and investigated. The hydrides of the halogen elements, such as hydrogen chloride (HC1), and the hydrides, water, sulphuretted hydrogen (H2S), and methane have been known for centuries. The phos phorus, arsenic and antimony hydrides analogous to ammonia have long been known, but the corresponding bismuth and polonium hydrides were discovered by F. Paneth only in 1918. The hydrides of tin and lead were discovered by Paneth two years later, and are probably analogous to methane, silane and germa nane, the last of which was discovered by E. Vogelen in 1902, though its composition as germanium tetrahydride was not estab lished till 20 years later by Paneth. It is a curious fact that the gaseous hydrides of the elements (except boron) are confined to groups IV., V., VI. and VII. of the periodic classification (q.v.) the hydrides of the alkali metals, the alkaline earth metals, the "rare earth" metals and of thorium and copper all being solids. The hydrides of chromium, cobalt, nickel and iron are also solids. Though hydrides of boron have been known for half a century, their composition was uncertain till 1913. when Alfred Stock com menced his extensive researches on the hydroborons. The simplest hydroboron is diborane, other hydrides having the formulae B i,,, and Owing to the presence of an odd number of hydrogen atoms in some of these hydrides, their formulation is impossible on the ordinarily accepted theories of valency. The existence of these compounds appears to demand the abandonment of the usual conception of valency, and the substitution of the actual number of electrons participating in each chemical bond. It is usual to postulate an invariable valency bond of two shared electrons, and it is almost certain that this type of bond is common to the majority of organic compounds. Evidence is accumulating, however, that this type of bond is not general in inorganic chemistry, particularly in the case of the more reactive non-ionized compounds, and a bond of a single shared electron appears to accord better with the experimental evidence.
Each element is described, together with its commoner com pounds, under its own heading.