The Roman numerals below the vertical columns refer to what is termed the "valency." An element capable of combining with or replacing one atom of hydrogen, or, in other words, of which the equivalent and atomic weight are identical (see p. 15), is termed a monad, or is said to be monovalent. Thus, 23 grams of sodium replaces I gram of hydrogen in water or in hydro gen chloride, to form hydroxide or chloride of sodium ; and as 23 is known to be the atomic weight of sodium from determinations of its specific heat, the atomic weight of sodium is expressed by the same number as its equivalent. It is therefore a monad. The element oxygen is a dyad or divalent, because, in water, two grams of hydrogen are combined with 1 6 grams of oxygen ; its equivalent is therefore 8. But oxygen is 16 times as heavy as hydro gen that is, a molecule of oxygen is 16 times as heavy as a molecule of hydrogen ; and as a molecule of each of these substances is believed to consist of two atoms, an atom of oxygen is 16 times as heavy as an atom of hydrogen. The atomic weight is therefore 1 6 ; but as
cylinders, on which the elements follow spiral lines, so that oxygen and fluorine, sulphur and chlorine, follow each other round the smaller cylinder, while selenium and bromine, tellurium and iodine, &c. , are conspicuous round the larger cylinder.
the equivalent is 8, the atomic weight is twice the equivalent. Hence the name " dyad" Similarly, there are tri valent elements, or triads ; tetravalent elements, or tetrads ; penta*valent elements, or pentads ; hexavalent elements, or hexads heptavalent elements, or heptads ; and possibly one octovalent element, or octad.
Valency of Elements
As elements may have more than one equivalent, so they may have more than one valency. Certain elements, however, so far as is known, possess only one valency ; examples of this are found in the lithium, the beryllium, and the boron columns. But the majority of elements exhibit more than one valency, according to circumstances. Thus, compounds of nitrogen are known possessing the formulae NO, NH g , NO 2 , and NH 4 C1, in which one atom of nitrogen is combined with one atom of dyad oxygen, and is therefore also a dyad ; with three atoms of monad hydrogen, and is accordingly a triad ; with two atoms of dyad oxygen, whence nitrogen is here a tetrad ; and with four atoms of monad hydrogen and one atom of monad chlorine in all, with five monads and in this case nitrogen must be accounted a pentad. The atomic weight of nitrogen is known from its density to be 14; and its equivalents in these compounds are respectively ~9 ^, ^, and ^. This peculiarity makes the classification of some of the elements a difficult task.
But there is an additional difficulty which meets us in attempting to ascribe the valency to an element. It is connected with what is known as the " structure " of compounds. As this subject will be frequently alluded to in succeeding chapters, enough will only be said here to give an idea of the problem which faces us in attempting a rational classification of the elements.
We are ignorant of the form of the atoms. It is true that various speculations have been made which may possibly lead to a true conception of their appearance and motions, but these are not sufficiently definite and sup ported by facts to require more than a passing allusion here. For all practical purposes, we are content, in default of a better conception, to regard atoms as spheres, (jfiarck and elastic, and compounds as formed by the juxtaposition of these spheres. That this conception is far from reality is more than probable, but it has to suffice. Certain deductions, however, may be drawn regarding the methods of combination of the atoms in the molecule. It is certain that molecules must occupy space of three dimensions ; but just as it is possible to represent solid objects on a plane surface by the help of perspective, so it is allowable to picture molecules as made up of atoms spread over a plane surface, until we find facts which demand space of three dimensions. We shall see later that in certain cases such solid models of molecules are necessary, but, as a rule, they can be dispensed with. And instead of attempting to picture the atoms as circles or projected spheres, the symbols alone will be employed. The fact of combination will be indicated by a dash uniting the atoms ; thus, a monad will have one, and only one, dash, proceeding from it ; a dyad, two ; a triad, three, and so on.