20 OUTLINES OF PHYSICAL CHEMISTRY
��The Constitution of the Molecule
Valency or Atomicity (KehuU)
A knowledge of the molecular weights of a large number of substances as well as of the number and nature of the atoms contained in the molecule, shows us that there are certain elements which unite with each other only in pairs, and in such a way that one atom of the one element combines with only one atom of the other. The power of attraction of the atoms of these elements does not extend to more than a single other atom. These elements are termed monatomic or monovalent.
There are other elements, a single atom of which is capable of uniting with two, three, four, or more mono- valent atoms ; their power of attraction extends to two, three, or four other atoms, and consequently they are termed divalent, trivalent, or tetravalent elements.
These peculiarities become evident by inspection of the following formulae, all of which represent volatile com- pounds :
I. HBr, HC1, HI, H 2 , Br 2 , Cl 2 , I 2 «
II. OH 2> 0C1 2> SH 2 , SeH 2 , TeH 2 , HgCl 2 , Hgl 2 .
III. BF 3 , BC1 3 , BBr 3 , NH 3 , PH 3 , PC1 3 , AsH 3 , AsCl 3 , Sb01 3 , BiCl 3 .
IV. 0H 4 , CC1 4 , SiF 4) SiCl 4 , TiCl 4 , VC1 4 , ZrCl 4 , Sn01 4 .
V. NbCl 5 , MoCl 5 , Ta01 s .
VI. WC1 6 .
Experiment also shows that in those compounds or elements belonging to any of the groups II. to VI. the monovalent atoms may be replaced by polyvalent atoms or groups of atoms. Thus
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