4 2

3 2

rest of the molecule differently from the other 3 hydrogen atoms; we therefore adopt the formula C,H,O,. H. Phosphorus pentachloride interacts with acetic acid; the interaction consists in the removal of one oxygen and one hydrogen atom from the molecule C,H,O, and the putting of one chlorine atom in their place; this interaction is thus expressed in an equation, C2H2O2+PC1=C2H2OC1+POCI12+HCl. From this we conclude that one of the oxygen atoms in the molecule CHO, directly interacts with an atom of hydrogen, and that the relation of this oxygen atom to the rest of the molecule is different from that of the other oxygen atom to the rest of the molecule; we therefore adopt the formula CHO. OH, and we express the interaction with phosphorus pentachloride thus,

2

C2H2O. OH + PCI, = C2H2O. Cl + POCI, + HCl.

3

2 3

3

If this interpretation of the mechanism of these changes is correct, then the group of atoms C2H2O is a compound radicle; this group remains unchanged when acetic acid interacts with phosphorus pentachloride, and it is common to the 2 molecules CH2O. OH and CH2O. Cl. If acetic acid has the formula CHO. OH the formula of sodium acetate is C.H2O. ONa : when this salt is mixed with solid caustic soda and the mixture is heated, sodium carbonate (Na,CO,) and methane (CH) are produced. Therefore in this change the atomic group C2H2O is decomposed; one of the carbon atoms and the oxygen atom are removed, and the remaining CH, combines with an atom of hydrogen to form CH1. The change is most simply represented thus,

CH,. CO. ONa + NaOH = Na ̧CO2+CH ̧H.

Because of this reaction we adopt for acetic acid the structural formula CH. CO. OH; and we say that the molecule of this acid is composed of the compound radicle CH, combined with the other compound radicles CO and OH. The

HO

developed structural formula H-C-C-O-H perhaps

makes this plainer.

H

Had we stopped this investigation after examining the interaction between PCI, and C,Н ̧O̟, we should have given

to acetic acid the structural formula C,H,O. OH, and we should have said that the molecule of this acid is composed of the compound radicles C2H2O and OH. Further investigation however obliges us to modify this conclusion, inasmuch as it shews that the atomic group C,H,O is itself composed of the simpler groups CH, and CO. The formula CH ̧. CO. OH expresses all that is expressed by the formula CH ̧O. OH, and it also suggests the interaction in which methane is produced from sodium acetate.

It appears then that a compound may have more than 371 one structural formula; that formula is the best which tells most about the characteristic reactions of the compound.

In Chap. XI. pars. 210 and 211 we glanced at the reactions of compounds of ammonia, NH,. We found that these reactions were analogous to those of the alkali potash, KOH; to bring out these analogies we wrote the formulae of the compounds produced by the interaction of an aqueous solution of ammonia with acids as compounds of the hypothetical compound radicle ammonium, NH,. The interpretation of these reactions given by the molecular and atomic theory is that in the molecule of an ammonium compound e.g. NHCl, NH.NO, (NH),SO,, (NH),CO, &c. we have always direct interaction between an atom of nitrogen and 4 atoms of hydrogen; in other words, we have the compound radicle or atomic group, NH..

As we speak of the valency of an atom in this or that molecule, meaning thereby the number of other atoms with which the specified atom directly interacts in the molecule, so we speak of the valency of an atomic group or compound radicle in a molecule. The atomic groups CH,, CH ̧‚ CH, &c. are monovalent; the group CH,OH is also monovalent; the group CO is divalent; and so on.

59

In par. 357 it was mentioned that an atom which combines 372 with one divalent atom to form a molecule is often regarded as thereby proved to be itself divalent. Thus the atom of oxygen is divalent because of the existence of the gaseous molecules OC1, and OH,; one atom of carbon combines with one atom of oxygen to form the gaseous molecule CO; therefore, it has been urged, the atom of carbon is divalent in the molecule CO. Again, one atom of carbon combines with 2 atoms of oxygen to form the gaseous molecule CO,; therefore, it is said, the atom of carbon is tetravalent in

M. E. C.

17

the molecule CO,.
generally used to express these statements.
These formulae are evidently based on
a meaning of
atomic valency different from that we have been giving
to this expression in preceding paragraphs. It is rather
difficult to grasp the exact meaning of the statement, 'the
atom of carbon is divalent in the molecule CO and tetra-
valent in the molecule CO,.' The statement seems to imply
that an atom of carbon is capable of directly combining
with either 2 or 4 hydrogen, fluorine, chlorine, bromine,
or iodine, atoms, or with that number of other atoms
which is equivalent to 2 or 4 atoms of chlorine, &c. The
statement seems to assert that one atom of oxygen is truly
equivalent sometimes to 2 atoms of hydrogen, or 2 atoms
of chlorine, &c. and sometimes to 4 atoms of hydrogen, &c.
But this assertion is scarcely capable of proof, because it
seems impossible to define the exact meaning to be given
to the expression equivalent to, as used with regard to

The formulae C=O and O=C=O are

atoms.

It has even been asserted that the atom of carbon is tetravalent in the molecule CO. If this is so, then one atom of oxygen is equivalent to 4 atoms of chlorine, &c. when oxygen and carbon combine to form the compound CO but 2 atoms of oxygen are equivalent to 4 atoms of chlorine, &c. when oxygen and carbon combine to form the compound CO,. We see here the extreme difficulty, if not impossibility, of giving an exact and invariable meaning to the expression equivalent to, as applied

to atoms.

373 It is generally the custom in writing structural formulae to represent each atom whose maximum valency is greater than one with as many lines proceeding from the symbol as correspond to the maximum valency of the atom. The atom of carbon, for instance, is generally represented with 4 lines proceeding from it, the atom of oxygen with 2 lines, and so Thus the structural formulae for ethylic aldehyde and acetic acid are generally written thus ;—

on.

Similarly the structural formulae of ethylene and the hypothetical isomeride ethylidene are generally written thus ;-H H

H

C, and H- C — C — H. The structural formula of

the hydrocarbon acetylene, CH,, is put into this form

2

H-C=C-H;

whereas the formula H-C-C-H would be used when the meaning given to valency is that explained in par, 360.

It would be out of place in an elementary book to discuss the possible meanings of these so called 'double bonds' and 'treble bonds.' In the opinion of several chemists they have done much to hinder the advance of chemistry, by leading chemists to trust in names, and in far-fetched analogies, instead of in realities, and in well established and accurately defined points of resemblance and difference. On the other hand the employment of 'double and treble linkings' or 'bonds' has some points in its favour. It continually reminds the chemist of the maximum valency of each atom, and by doing this it suggests the possibility of reactions.

represents each carbon atom as directly interacting with only 3 other atoms, but formula (2) tells us that a carbon atom can directly interact with 4 other atoms, and hence suggests the possibility of adding 2 monovalent atoms to the molecule

CH. So the formula for ethylic aldehyde H-C-C

HO

visibly suggests the possibility of putting 2 monovalent atoms, e.g. Cl,, in place of the atom of oxygen.

The formulae C C and H C C of course suggest

the same reactions as the formulae with double bonds, if we remember that the maximum valency of the carbon atom is 4, and that of the oxygen atom is 2. These formulae have the great advantage over those with double or treble bonds that they are based on a definite hypothesis regarding atomic valency. 374 The essential part of the hypothesis of valency is the conception of direct action and reaction between each atom in a molecule and a limited number of other atoms. As the whole molecule is held together by the mutual interactions of the atoms, there probably is what we may call indirect action and reaction between all the atoms which constitute the molecule. The hypothesis gives us a definition of the maximum valency of an atom, as the maximum number of monovalent atoms (i.e. atoms of H, F, Cl, Br, or I) with which the given atom directly interacts (i.e. in these cases combines) in any molecule; and it teaches that the specified atom never directly interacts with a greater number of other atoms, whatever be their valencies, than is expressed by the maximum valency as thus defined.

375

The hypothesis of valency is meaningless apart from the theory of atoms and molecules; it is based on this theory, and all the results gained by using it are expressed in the language of the theory.

The theory of atoms and molecules is strictly applicable, at present, only to gases; therefore the hypothesis of valency, and all the terms to which it has given birth, are strictly applicable, at present, only to gases. But just as we made use of the molecular and atomic theory as a general guide in studying the chemical changes which occur among liquid and solid substances, so may we make use of the hypothesis of valency, provided we exercise sufficient caution, as a general guide in attempts to learn something regarding the structure of those aggregations of atoms which form the reacting weights of solid and liquid substances. But it would be going too far afield to attempt to indicate here even the lines on which the hypothesis of valency may probably be