CH2O

ch2o, and its molecular formula is c2h4o2, or twice ch2o. in addition to empirical and molecular formulae, chemists are in the habit of employing various kinds of rational formulae, called structural, constitutional or graphic formulae, &c., which not only express the molecular composition of the compounds to which they apply, but also embody certain assumptions as to the manner in which the constituent atoms are arranged, and convey more or less information with regard to the nature of the compound itself, viz. the class to which it belongs, the manner in which it is formed, and the behaviour it will exhibit under various circumstances. before explaining these formulae it will be necessary, however, to consider the differences in combining power exhibited by the various elements. _valency._--it is found that the number of atoms of a given element, of chlorine, for example, which unite with an atom of each of the other elements is very variable. thus, hydrogen unites with but a single atom of chlorine, zinc with two, boron with three, silicon with four, phosphorus with five and tungsten with six. those elements which are equivalent in combining or displacing power to a single atom of hydrogen are said to be _univalent_ or _monad_ elements; whilst those which are equivalent to two atoms of hydrogen are termed bivalent or dyad elements; and those equivalent to three, four, five or six atoms of hydrogen triad, tetrad, pentad or hexad elements. but not only is the combining power or valency (atomicity) of the elements different, it is also observed that one element may combine with another in several proportions, or that its valency may vary; for example, phosphorus forms two chlorides represented by the formulae pcl3 and pcl5, nitrogen the series of oxides represented by the formulae n2o, no, (n2o3), n2o4,