Metals. Although each metal is considered in a separate article, there are various points regarding the general physical and chemical characters of these bodies, and the method of classifying them, which require notice. A metal, from the chemical point of view, is an element which can replace hydrogen in an acid and thus form a salt. Hydrogen itself is, chemically, considered to be a metal. Those elements which are non-metallic in this sense are called metalloids.
Amongst the chief chemical properties of metals we notice their strong affinities to certain of the non-metallic elements. All the metals, without exception, combine with oxygen, sulphur, and chlorine, and often in several proportions, forming oxides, sulphides (formerly termed sulphurets), and chlorides. Many of them combine with bromine, iodine, and fluorine. The other compounds of this nature, excepting carbide (formerly carburet) of iron, or steel, and the hydrides of arsenic and antimony (commonly known as arseniuretts and antimoniuuretts hydrogen), which are of importance in toxicology, may be passed over without notice. The metallic oxides are, without exception, solid bodies, white or coloured, and usually present an earthy appearance. Hence the old name of metallic calx applied to these oxides. Those oxides which are termed basic possess the property of directly uniting with the so-called oxy-acids (such as sulphuric, nitric, carbonic, and silicic acid), and of forming new chemical compounds of the second order, termed Salts (q.v.). The compounds of the metals with chlorine, iodine, bromine, and fluorine—such, for instance, as chloride of sodium, or common salt, —are termed haloid salts. The same metal may often combine both with chlorine and with oxygen in more than one proportion. For example, we have subchloride of mercury, ; suboxide of mercury, ; chloride of mercury, ; oxide of mercury, . For the compounds of the metals with sulphur, see SULPHUR.
The following are the most important of the physical properties of the metals: (1) All metals, unless when they are in a finely-pulverised form, exhibit more or less of the characteristic lustre termed metallic. Two of the non-metallic elements, iodine and carbon, in some forms also present a metallic lustre. (2) All metals are good conductors of heat and electricity, although in very unequal degrees. (3) With the exception of mercury, all the metals are solid at ordinary temperatures. With the exception of gold, copper, calcium, and strontium, the metals are, when light is only once reflected from them, more or less white, with a tendency to blue or gray. Most of them have been obtained in crystals, and probably all of them are capable of crystallising under certain conditions. (4) Metals are remarkable for their opacity, except when they are chemically reduced to extremely thin films. (5) All the metals are fusible, although the temperatures at which they assume the fluid form are very different (see MELTING-POINT); and some of them, as mercury, arsenic, cadmium, zinc, &c., are also volatile. (6) Great weight, or a high specific gravity, is popularly but erroneously regarded as a characteristic of a metal; while platinum, osmium, and iridium (the heaviest bodies known in nature) are more than twenty times as heavy as water, lithium, potassium, and sodium are actually lighter than that fluid. (7) Great differences are observable in the hardness, brittleness, and tenacity of metals. While potassium and sodium may be kneaded with the finger, and lead may be marked by the finger-nail, most of them possess a considerable degree of hardness. Antimony, arsenic, and bismuth are so brittle that they may be easily pulverised in a mortar; while others, as iron, gold, silver, and copper, require great force for their disintegration. Taking iron and lead as representing the two extremes of tenacity, it is found that an iron wire will bear a weight twenty-six times as heavy as a leaden wire of the same diameter. See DUCTILITY, MALLEABILITY.
Metals enter into combination with one another when they are fused together, and such combinations are termed Alloys (q.v.), unless when mercury is one of the combining metals, in which case the resulting compound is termed an Amalgam (q.v.). It is doubtful whether all alloys are true chemical compounds. Definite compounds of the metals with each other do, however, certainly exist, and are sometimes found native, as, for example, the crystallised silver and mercury compound represented by the formula .
In consequence of their strong affinities for the metalloids the metals are seldom found in a free or uncombined state, even in the inorganic kingdom, and never in animals or plants. The more common metals, in consequence of their strong affinity for oxygen and sulphur, are very rarely met with in the uncombined state; but some of those which are less abundant, such as gold, silver, and platinum, are found uncombined, in which case the terms native and virgin are applied to them; and other metals, as mercury and copper, occur both in a free and in a combined state. Many native alloys are found, but the ordinary sources of the metals are oxides, sulphides, chlorides, and carbonates, sulphates, and other salts. These are termed the ores of the metals. The methods of obtaining the metals from their various ores fall under the head of METALLURGY.
Various classifications of the metals have been suggested by different chemists. The following is probably one of the most convenient:
(I.) The Light Metals, subdivided into—
(1) The metals of the alkalis—viz. potassium, sodium, caesium, rubidium, lithium.
(2) The metals of the alkaline earths—viz. barium, strontium, calcium, magnesium.
(3) The metals of the true earths—viz. aluminium, glucinum, zirconium, yttrium, erbium, terbium, thorium, cerium, lanthanum, didymium.
(II.) The Heavy Metals, subdivided into—
(1) Metals whose oxides form powerful bases—viz. iron, manganese, chromium, nickel, cobalt, zinc, cadmium, lead, bismuth, copper, uranium, thallium.
(2) Metals whose oxides form weak bases or acids—viz. arsenic, antimony, titanium, tantalum, niobium (or columbium), tungsten, molybdenum, tin, vanadium, osmium.
(3) Metals whose oxides are reduced by heat—noble metals—viz. mercury, silver, gold, platinum, palladium, iridium, ruthenium, rhodium, osmium. (Several of the rare metals are here omitted.)
Another classification is that by which the metals are arranged in six groups, each group being named after a metal which possesses the common characters in a well-marked degree: viz. (1) the sodium group, (2) the calcium, (3) the iron, (4) the copper, (5) the platinum, and (6) the antimony group.