represents the decomposition of ammonia gas into nitrogen and hydrogen
gases by the electric spark, and it not only conveys the information
that a certain relative weight of ammonia, consisting of certain
relative weights of hydrogen and nitrogen, is broken up into certain
relative weights of hydrogen and nitrogen, but also that the nitrogen
will be contained in half the space which contained the ammonia, and
that the volume of the hydrogen will be one and a half times as great as
that of the original ammonia, so that in the decomposition of ammonia
the volume becomes doubled.
Formulae which merely express the relative number of atoms of the
different elements present in a compound are termed _empirical
formulae_, and the formulae of all compounds whose molecular weights are
undetermined are necessarily empirical. The _molecular formula_ of a
compound, however, is always a simple multiple of the empirical formula,
if not identical with it; thus, the empirical formula of acetic acid is
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,
N2O5, molybdenum forms the chlorides MoCl2, MoCl3, MoCl4, MoCl5,
MoCl6(?), and tungsten the chlorides WCl2, WCl4, WCl5, WCl6.
In explanation of these facts it is supposed that each element has a
certain number of "units of affinity," which may be entirely, or only in
part, engaged when it enters into combination with other elements; and
in those cases in which the entire number of units of affinity are not
engaged by other elements, it is supposed that those which are thus
disengaged neutralize each other, as it were. For example, in phosphorus
pentachloride the five units of affinity possessed by the phosphorus
atom are satisfied by the five monad atoms of chlorine, but in the
trichloride two are disengaged, and, it may be supposed, satisfy each
other. Compounds in which all the units of affinity of the contained
elements are engaged are said to be _saturated_, whilst those in which
the affinities of the contained elements are not all engaged by other
elements are said to be _unsaturated_. According to this view, it is
necessary to assume that, in all unsaturated compounds, two, or some
even number of affinities are disengaged; and also that all elements
which combine with an even number of monad atoms cannot combine with an
odd number, and vice versa,--in other words, that the number of units of
affinity active in the case of any given element must be always either
an even or an odd number, and that it cannot be at one time an even and
at another an odd number. There are, however, a few remarkable
exceptions to this "law." Thus, it must be supposed that in nitric
oxide, NO, an odd number of affinities are disengaged, since a single
atom of dyad oxygen is united with a single atom of nitrogen, which in
all its compounds with other elements acts either as a triad or pentad.
When nitric peroxide, N2O4, is converted into gas, it decomposes, and at
about 180 deg. C. its vapour entirely consists of molecules of the
composition NO2; while at temperatures between this and 0 deg. C. it
consists of a mixture in different proportions of the two kinds of
molecules, N2O4 and NO2. The oxide NO2 must be regarded as another
instance of a compound in which an odd number of affinities of one of
the contained elements are disengaged, since it contains two atoms of
dyad oxygen united with a single atom of triad or pentad nitrogen.
Again, when tungsten hexachloride is converted into vapour it is
decomposed into chlorine and a pentachloride, having a normal vapour
density, but as in the majority of its compounds tungsten acts as a
hexad, we apparently must regard its pentachloride as a compound in
which an odd number of free affinities are disengaged. Hitherto no
explanation has been given of these exceptions to what appears to be a
law of almost universal application, viz. that the sum of the units of
affinity of all the atoms in a compound is an even number.
The number of units of affinity active in the case of any particular
element is largely dependent, however, upon the nature of the element or
elements with which it is associated. Thus, an atom of iodine only
combines with one of hydrogen, but may unite with three of chlorine,
which never combines with more than a single atom of hydrogen; an atom
of phosphorus unites with only three atoms of hydrogen, but with five of
chlorine, or with four of hydrogen and one of iodine; and the chlorides
corresponding to the higher oxides of lead, nickel, manganese and
arsenic, PbO2, Ni2O3, MnO2 and AS2O5 do not exist as stable compounds,
but the lower chlorides, PbCl2, NiCl2, MnCl2 and AsCl3, are very stable.
The valency of an element is usually expressed by dashes or Roman
numerals placed on the right of its symbol, thus: H', O'', B''', C^{IV},
P^V, Mo^{VI}; but in constructing graphic formulae the symbols of the
elements are written with as many lines attached to each symbol as the
element which it represents has units of affinity.
The periodic law (see ELEMENT) permits a grouping of the elements
according to their valency as follows:--Group O.: helium, neon, argon,
krypton and xenon appear to be devoid of valency. Group I.: the alkali
metals Li, Na, K, Rb, Cs, and also Ag, monovalent; Cu, monovalent and
divalent; Au, monovalent and trivalent. Group II.: the alkaline earth
metals Ca, Sr, Ba, and also Be (Gl), Mg, Zn, Cd, divalent; Hg,
monovalent and divalent. Group III.: B, trivalent; Al, trivalent, but
possibly also tetra- or penta-valent; Ga, divalent and trivalent; In,
mono-, di- and tri-valent; Tl, monovalent and trivalent. Group IV.: C,
Si, Ge, Zr, Th, tetravalent; Ti, tetravalent and hexavalent; Sn, Pb,
divalent and tetravalent; Ce, trivalent and tetravalent. Group V.: N,
trivalent and pentavalent, but divalent in nitric oxide; P, As, Sb, Bi,
trivalent and pentavalent, the last being possibly divalent in BiO and
BiCl2. Group VI.: O, usually divalent, but tetravalent and possibly
hexavalent in oxonium and other salts; S, Se, Te, di-, tetra- and
hexa-valent; Cr, di-, tri- and hexa-valent; Mo, W, di-, tri-, tetra-,
penta- and hexa-valent. Group VII.: H(?), monovalent; the halogens F,
Cl, Br, I, usually monovalent, but possibly also tri- and pentavalent;
Mn, divalent and trivalent, and possibly heptavalent in permanganates.
Group VIII.: Fe, Co, divalent and trivalent; Ni, divalent; Os, Ru,
hexavalent and octavalent; Pd, Pt, divalent and tetravalent; Ir, tri-,
tetra- and hexa-valent. (See also VALENCY.)
_Constitutional Formulae._--Graphic or constitutional formulae are
employed to express the manner in which the constituent atoms of
compounds are associated together; for example, the trioxide of sulphur
is usually regarded as a compound of an atom of hexad sulphur with three
atoms of dyad oxygen, and this hypothesis is illustrated by the graphic
formula
O
//
O=S
\\
O .
When this oxide is brought into contact with water it combines with it
forming sulphuric acid, H2SO4.
In this compound only two of the oxygen atoms are wholly associated with
the sulphur atom, each of the remaining oxygen atoms being united by one
of its affinities to the sulphur atoms, and by the remaining affinity to
an atom of hydrogen; thus--
H.O O
\ //
S
/ \\
H.O O .
The graphic formula of a sulphate is readily deduced by remembering that
the hydrogen atoms are partially or entirely replaced. Thus acid sodium
sulphate, normal sodium sulphate, and zinc sulphate have the formulae
Na.O O Na.O O O O
\ // \ // / \ //
S S Zn S
/ \\ / \\ \ / \\
H.O O , Na.O O , O O .
Again, the reactions of acetic acid, C2H4O2, show that the four atoms of
hydrogen which it contains have not all the same function, and also that
the two atoms of oxygen have different functions; the graphic formula
which we are led to assign to acetic acid, viz.
O
H //
H.C--C
. \
H O.H
serves in a measure to express this, three of the atoms of hydrogen
being represented as associated with one of the atoms of carbon, whilst
the fourth atom is associated with an atom of oxygen which is united by
a single affinity to the second atom of carbon, to which, however, the
second atom of oxygen is united by both of its affinities. It is not to
be supposed that there are any actual _bonds_ of union between the
atoms; graphic formulae such as these merely express the hypothesis that
certain of the atoms in a compound come directly within the sphere of
attraction of certain other atoms, and only indirectly within the sphere
of attraction of others,--an hypothesis to which chemists are led by
observing that it is often possible to separate a group of elements from
a compound, and to displace it by other elements or groups of elements.
Rational formulae of a much simpler description than these graphic
formulae are generally employed. For instance, sulphuric acid is usually
represented by the formula SO2(OH)2, which indicates that it may be
regarded as a compound of the group SO2 with twice the group OH. Each of
these OH groups is equivalent in combining or displacing power to a
monad element, since it consists of an atom of dyad oxygen associated
with a single atom of monad hydrogen, so that in this case the SO2 group
is equivalent to an atom of a dyad element. This formula for sulphuric
acid, however, merely represents such facts as that it is possible to
displace an atom of hydrogen and an atom of oxygen in sulphuric acid by
a single atom of chlorine, thus forming the compound SO3HCl; and that by
the action of water on the compound SO2Cl2 twice the group OH, or water
minus an atom of hydrogen, is introduced in place of the two monad atoms
of chlorine--
SO2Cl2 + 2HOH = SO2(OH)2 + 2HCl.
Water. Sulphuric acid.
Constitutional formulae like these, in fact, are nothing more than
symbolic expressions of the character of the compounds which they
represent, the arrangement of symbols in a certain definite manner being
understood to convey certain information with regard to the compounds
represented.
Groups of two or more atoms like SO2 and OH, which are capable of
playing the part of elementary atoms (that is to say, which can be
transferred from compound to compound), are termed compound radicals,
the elementary atoms being simple radicals. Thus, the atom of hydrogen
is a monad simple radical, the atom of oxygen a dyad simple radical,
whilst the group OH is a monad compound radical.
It is often convenient to regard compounds as formed upon certain types;
alcohol, for example, may be said to be a compound formed upon the water
type, that is to say, a compound formed from water by displacing one of
the atoms of hydrogen by the group of elements C2H5, thus--
{ H { C2H5
O { O {
{ H { H
Water Alcohol.
Constitutional formulae become of preponderating importance when we
consider the more complicated inorganic and especially organic
compounds. Their full significance is treated in the section of this
article dealing with organic chemistry, and in the articles ISOMERISM
and STEREO-ISOMERISM.
_Chemical Action._--Chemical change or chemical action may be said to
take place whenever changes occur which involve an alteration in the
composition of molecules, and may be the result of the action of agents
such as heat, electricity or light, or of two or more elements or
compounds upon each other.
Three kinds of changes are to be distinguished, viz. changes which
involve combination, changes which involve decomposition or separation,
and changes which involve at the same time both decomposition and
combination. Changes of the first and second kind, according to our
views of the constitution of molecules, are probably of very rare
occurrence; in fact, chemical action appears almost always to involve
the occurrence of both these kinds of change, for, as already pointed
out, we must assume that the molecules of hydrogen, oxygen and several
other elements are diatomic, or that they consist of two atoms. Indeed,
it appears probable that with few exceptions the elements are all
compounds of similar atoms united together by one or more units of
affinity, according to their valencies. If this be the case, however, it
is evident that there is no real distinction between the reactions which
take place when two elements combine together and when an element in a
compound is displaced by another. The combination, as it is ordinarily
termed, of chlorine with hydrogen, and the displacement of iodine in
potassium iodide by the action of chlorine, may be cited as examples; if
these reactions are represented, as such reactions very commonly are, by
equations which merely express the relative weights of the bodies which,
enter, into reaction, and of the products, thus--
H + Cl = HCl
Hydrogen. Chlorine. Hydrochloric acid.
KI + Cl = KCl + I
Potassium iodide. Chlorine. Potassium chloride. Iodine.
they appear to differ in character; but if they are correctly
represented by molecular equations, or equations which express the
relative number of molecules which enter into reaction and which result
from the reaction, it will be obvious that the character of the reaction
is substantially the same in both cases, and that both are instances of
the occurrence of what is ordinarily termed double decomposition--
H2 + Cl2 = 2HCl
Hydrogen. Chlorine. Hydrochloric acid.
2KI + Cl2 = 2KCl + I2.
Potassium iodide. Chlorine. Potassium chloride. Iodine.
In all cases of chemical change energy in the form of heat is either
developed or absorbed, and the amount of heat developed or absorbed in a
given reaction is as definite as are the weights of the substance
engaged in the reaction. Thus, in the production of hydrochloric acid
from hydrogen and chlorine 22,000 calories are developed; in the
production of hydrobromic acid from hydrogen and bromine, however, only
844O calories are developed; and in the formation of hydriodic acid from
hydrogen and iodine 6040 calories are absorbed.
This difference in behaviour of the three elements, chlorine, bromine
and iodine, which in many respects exhibit considerable resemblance, may
be explained in the following manner. We may suppose that in the
formation of gaseous hydrochloric acid from gaseous chlorine and
hydrogen, according to the equation
H2 + Cl2 = HCl + HCl,
a certain amount of energy is expended in separating the atoms of
hydrogen in the hydrogen molecule, and the atoms of chlorine in the
chlorine molecule, from each other; but that heat is developed by the
combination of the hydrogen atoms with the chlorine atoms, and that, as
more energy is developed by the union of the atoms of hydrogen and
chlorine than is expended in separating the hydrogen atoms from each
other and the chlorine atoms from one another, the result of the action
of the two elements upon each other is the development of heat,--the
amount finally developed in the reaction being the difference between
that absorbed in decomposing the elementary molecules and that developed
by the combination of the atoms of chlorine and hydrogen. In the
formation of gaseous hydrobromic acid from liquid bromine and gaseous
hydrogen--
H2 + Br2 = HBr + HBr,
in addition to the energy expended in decomposing the hydrogen and
bromine molecules, energy is also expended in converting the liquid
bromine into the gaseous condition, and probably less heat is developed
by the combination of bromine and hydrogen than by the combination of
chlorine and hydrogen, so that the amount of heat finally, developed is
much less than is developed in the formation of hydrochloric acid.
Lastly, in the production of gaseous hydriodic acid from hydrogen and
solid iodine--