Tetravalency of Oxygen. 223 



siderations of valency or atomicity. But it may subsequently 

 affect the questions as to the actual number of hydrogen and 

 chlorine atoms in a molecule of hydrogen chloride, and the 

 discussion of what Professor Armstrong calls " residual 

 affinity." The atomic weight — that is, ratio-atomic weight — 

 of oxygen is in this sense 16. But it is well to bear in mind 

 that the number can, if necessary, be taken in the more natural 

 sense of a " combining proportion " or combining weight, and 

 that this number 16 is not indissolubly bound up with the 

 statement that the oxygen atom is diatomic or divalent, as is 

 sometimes supposed. Indeed, Professor Odling, in his historic 

 paper* on the weight of the oxygen atom, demonstrated that 

 = 16, and not 8 as then usually taken if N = 14, as was 

 universally accepted whether in a strictly atomic sense or not. 



Taking, then, equal volumes of these three gases, it is a 

 fact that we have one, two, and three unit weights and volumes 

 combined with the chemical unit weights of CI, 0, and N. 

 Apart even from Avogadro's law, the chemist would, adopting 

 the atomic theory, say that " atoms " of chlorine, oxygen, 

 and nitrogen respectively combined with one, two, and three 

 atoms of hydrogen in these compounds ; and hence that these 

 atoms were mono-, di-, and tri-atomic respectively. In the de- 

 velopments of the substitution- and type-theories, and above 

 all in the development of the hydrocarbon theory, based 

 upon the very strikingly constant u tetratomicity of carbon," 

 this theory has been so useful that there has been a strong 

 school of chemists to maintain this atomicity as a constant 

 property — " a property inherent in the nature of atoms." f 



The word "atomicity" is thus naturally associated with 

 some inherent and occult property of the atom or some force 

 centering from it. Since it has become more generally 

 recognized that the atomicity of many elements seems to 

 vary when either (1) the chemical conditions of A and B are 

 altered, or (2) other elements are chemically associated with A 

 or B, the word " atomicity " has wisely gone out of fashion. 

 More strictly speaking, it is the phrase " the atom of oxygen 

 is diatomic " which is disappearing, because physicists (e. g. 

 Prof. J. J. Thomson) write about " diatomic molecules," 

 meaning molecules which contain two chemical atoms. Thus 

 it is now said that " the molecule of oxygen is diatomic/'' and 

 that " the atom of oxygen is divalent," or, most briefly, that 

 " oxygen is a dyad." The shorthand form remains distinct ; it 

 is 2 for the diatomic molecule, but 0" or O 11 for the divalent 

 atom. 



* Journ. Cliem. Soc. London, vol. xi. 

 t Wurtz, ' Atomic Theory/ p. 224. 



