CHAPTER III CHEMICAL NOMENCLATURE THE names which have been given to the various elementary forms of matter are not based upon any scientific system. The names of some have their origin in mythology. Others have received names which are indicative of some characteristic property, while those of several bear reference to some special circumstance connected with their discovery. It has been the custom in modern times to distinguish metals from non-metals by applying to the former names ending in the letters um, and consequently such metals as are of 'more recent discovery all have names with this termination. The common metals, however, which have been known since earlier times, such as gold, silver, tin, and copper, keep their old names. The two elements selenium and tellurium were at the time of their discovery thought to be metals, and they consequently received names with the terminal um; these substances strongly resemble metals in many of their physical properties, but in their chemical relations they are so closely similar to the non-metal sulphur, that they are by general consent classed among the non-metals; they are examples of those elements which are distinguished as metalloids. On this account selenium is by some chemists termed selenion. In naming chemical compounds, the chemist endeavours that the names employed shall not only serve to identify the substances, but shall as far as possible indicate their composition. The simplest chemical compounds are those composed of only two different elements; such are spoken of as binary compounds,* and their names are made up of the names of the two elements composing them, thus * This expression is now sometimes used in a somewhat modified sense. Thus in the language of the ionic theory (p. 106) the term binary compound is used to denote a substance which dissociates into two ions, quite irrespective of the number of elements it may contain. It is to be regretted that under these circumstances a new word was not coined to denote the newer idea. The compound formed by the chemical union of Hydrogen with sulphur is called hydrogen sulphide. It continually happens, however, that the same two elements combine together in more than one proportion, giving rise to as many different compounds, in which case it becomes necessary to so modify the names that each of the compounds may be distinguished. This is accomplished by the use of certain terminal letters or of certain prefixes; for example, the element phosphorus combines with chlorine in two proportions, forming two different compounds-in one the molecules contain one atom of phosphorus united to three atoms of chlorine, in the other the molecules consist of one atom of phosphorus associated with five of chlorine. These two compounds may be distinguished in the following ways :— I atom of phosphorus with 3 atoms of chlorine forms phosphorous chloride. phosphoric chloride. I " or 5 " 1 atom of phosphorus with 3 atoms of chlorine forms phosphorus trichloride. phosphorus pentachloride. I 5 The latter method of distinction is the more general, thus 1 atom of sulphur with 2 atoms of oxygen forms sulphur dioxide. I atom of carbon with 1 atom of oxygen forms carbon monoxide. Occasionally the prefixes sub and proto are employed to denote these differences of composition, but their use is more limited, and is becoming out of vogue. When more than two compounds are formed by the union of the same two elements, the additional prefixes hypo, under, and per, over, are sometimes used. In a considerable number of instances the systematic names of familiar compounds give way to the vulgar or common names by which they are known, thus Binary compounds that are formed by the union of elements with oxygen are called the oxides of those elements. Certain of these oxides are capable of entering into combination with water, giving rise to substances known as acids; such oxides are distinguished as acid-forming oxides, or acidic oxides. They are also sometimes termed anhydrides. All the non-metallic elements, except hydrogen and the members of the argon group, form oxides of this order, and the acids derived from them are known as the oxy-acids. Certain other oxides also unite with water, but give rise to compounds known as hydroxides. When such oxides, which are all derived from the metallic elements, are brought into contact with acids, chemical union takes place, and a compound termed a salt is formed. Such oxides are distinguished as salt-forming or basic oxides. There are also oxides which are neither acidic nor basic. The names of oxy-acids are derived from the name of the particular oxide from which they are formed, thus Carbon dioxide gives carbonic acid. When the same element forms two acid-forming oxides, the terminals ic and ous are applied to the acids to denote respectively the one with the greater and the less proportion of oxygen, thus Sulphur trioxide gives sulphuric acid. Nitrogen trioxide gives nitrous acid. When more than two such acids are known, the additional prefixes hypo or per are made use of. Thus persulphuric acid denotes an acid containing the highest quantity of oxygen, while hyponitrous acid stands for an acid containing less oxygen than is present in nitrous acid. There is a class of binary compounds formed by the combination of a large number of the elements with sulphur; these are known as sulphides. Certain of these sulphides are also capable of forming acids which are analogous in their constitution to oxy-acids, but in which the oxygen atoms are substituted by atoms of sulphur. These acids are known as thio acids (sometimes sulpho acids), and the same system of nomenclature is adopted to distinguish these thus we have thio-arsenious acid, thio-arsenic acid, denoting B respectively the acid with the smaller and the larger proportion of sulphur. It was at one time believed that all acids contained oxygen, that indeed this element was essential to an acid. The name oxygen indicates this belief, the word signifying "the acid-producer." This view is now seen to have been incorrect, for many acids are known in which oxygen is not one of the constituents. Thus the elements fluorine, chlorine, bromine, and iodine, which constitute the so-called Halogen group of elements, each combines with hydrogen, giving rise respectively to hydrofluoric, hydrochloric, hydriodic, and hydrobromic acids. All known acids contain hydrogen as one of their constituents. As already stated, when chemical action takes place between an acid and a base * a salt is formed. Oxy-acids in this way give rise to oxy-salts, thio-acids to thio-salts, and halogen acids to haloid salts. The latter salts being binary compounds, their names are given according to the system already explained, such, for example, as calcium fluoride, sodium chloride, potassium bromide, silver iodide. In the case of the oxy-salts and thio-salts, the names are made up from the names of the acid and of the metal contained in the base, with the addition of certain distinctive suffixes: thus if the acid be one whose name carries the terminal ous, its salts will be * The word base is unfortunately employed by different chemists in different senses, so that it is scarcely possible to give a precise definition of it. Originally, no doubt, the term was employed simply to denote the idea of foundation, and was applied to the metal or the oxide of the metal entering into the composition of a salt; which being the more tangible constituent was thus regarded as the more important one, or the basis of the salt. At the present day the word base is used in INORGANIC chemistry chiefly to denote that class of compounds described on page 17 as hydroxides, while the oxides from which these hydroxides are derived are spoken of as basic oxides. Besides this class, it includes ammonia and a few other compounds which like ammonia are not derived from metallic oxides. The ORGANIC chemist, on the other hand, regards ammonia as the true type of a base; and all organic compounds which can be regarded as "derivatives" of ammonia are called bases. Not only so, but the term is even extended so as to include similar "derivatives" of the phosphorus, arsenic and antimony analogues of ammonia, thus giving rise to the expressions nitrogen bases, phosphorus bases, &c. Again, in the language of the modern theory of ionic dissociation, a base is defined as a compound in which the only negative ions are the hydroxide ions (page 107). This definition includes the class of hydroxides above mentioned, but does not include ammonia gas. distinguished by the suffix ite, while the names of the salts derived from acids whose names end in ic are terminated by the letters The formation of a salt by the action of an acid upon a base is due to the redistribution of the atoms composing the molecules of the two compounds, in such a manner that some or all of the hydrogen atoms in the acid molecules exchange places with certain metallic atoms from the molecules of the base. Acids which contain only one atom of hydrogen so capable of becoming exchanged for a metal are termed mono-basic acids; those with two, three, or four such hydrogen atoms are distinguished respectively as di-basic, tri-basic, and tetra-basic acids. If the whole of the displaceable hydrogen in an acid becomes replaced by the base, the salt formed is known as a normal salt. On the other hand, when only a portion of the hydrogen atoms is displaced by the base, the salt is distinguished as an acia salt. Thus sulphuric acid contains two atoms of hydrogen in its molecule (associated with one of sulphur and four of oxygen); if both the hydrogen atoms are exchanged for potassium, the salt obtained is normal potassium sulphate, and when only one is so replaced the salt is known as acid potassium sulphate. By the term acid salt, therefore, must be understood not a substance having the familiar properties of an acid, such as a sour taste and the power to redden litmus, but a salt in which one or more of the hydrogen atoms of the original acid are still left in the molecule.* It is quite true that some of the salts of this class do possess acid qualities and will redden litmus, but this is due to what may be regarded as merely the accidental circumstance of the acidic portion of the molecule being derived from a strong acid. Many substances belonging to the class of acid salts are perfectly neutral in their behaviour towards litmus, while, on the other hand, some are strongly alkaline. For example, acid potassium sulphate is acid * Some chemists prefer to regard the acids themselves as the hydrogen salts; accordingly they apply to nitric acid, sulphuric acid, nitrous acid, sulphurous acid, &c., the names hydrogen nitrate, hydrogen sulphate, hydrogen nitrite, hydrogen sulphite, &c., respectively. |