VALENCE
~Definition of valence.~ A study of the formulas of various binary
compounds shows that the elements differ between themselves in the
number of atoms of other elements which they are able to hold in
combination. This is illustrated in the formulas
HCl, H_{2}O, H_{3}N, H_{4}C.
(hydrochloric acid) (water) (ammonia) (marsh gas)
It will be noticed that while one atom of chlorine combines with one
atom of hydrogen, an atom of oxygen combines with two, an atom of
nitrogen with three, one of carbon with four. The number which expresses
this combining ratio between atoms is a definite property of each
element and is called its _valence_.
DEFINITION: _The valence of an element is that property which determines
the number of the atoms of another element which its atom can hold in
combination._
~Valence a numerical property.~ Valence is therefore merely a numerical
relation and does not convey any information in regard to the intensity
of the affinity between atoms. Judging by the heat liberated in their
union, oxygen has a far stronger affinity for hydrogen than does
nitrogen, but an atom of oxygen can combine with two atoms only of
hydrogen, while an atom of nitrogen can combine with three.
~Measure of valence.~ In expressing the valence of an element we must
select some standard for comparison, just as in the measurement of any
other numerical quantity. It has been found that an atom of hydrogen is
never able to hold in combination more than one atom of any other
element. Hydrogen is therefore taken as the standard, and other elements
are compared with it in determining their valence. A number of other
elements are like hydrogen in being able to combine with at most one
atom of other elements, and such elements are called _univalent_. Among
these are chlorine, iodine, and sodium. Elements such as oxygen,
calcium, and zinc, which can combine with two atoms of hydrogen or other
univalent elements, are said to be _divalent_. Similarly, we have
_trivalent, tetravalent, pentavalent_ elements. None have a valence of
more than 8.
~Indirect measure of valence.~ Many elements, especially among the metals,
do not readily form compounds with hydrogen, and their valence is not
easy to determine by direct comparison with the standard element. These
elements, however, combine with other univalent elements, such as
chlorine, and their valence can be determined from the compounds so
formed.
~Variable valence.~ Many elements are able to exert different valences
under differing circumstances. Thus we have the compounds Cu_{2}O and
CuO, CO and CO_{2}, FeCl_{2} and FeCl_{3}. It is not always possible to
assign a fixed valence to an element. Nevertheless each element tends to
exert some normal valence, and the compounds in which it has a valence
different from this are apt to be unstable and easily changed into
compounds in which the valence of the element is normal. The valences of
the various elements will become familiar as the elements are studied in
detail.
~Valence and combining ratios.~ When elements combine to form compounds,
the ratio in which they combine will be determined by their valences. In
those compounds which consist of two elements directly combined, the
union is between such numbers of the two atoms as have equal valences.
Elements of the same valence will therefore combine atom for atom.
Designating the valence of the atoms by Roman numerals placed above
their symbols, we have the formulas
II II II III III IV IV
HCl, ZnO, BN, CSi.
A divalent element, on the other hand, will combine with two atoms of a
univalent element. Thus we have
II II II II
ZnCl_{2} and H_{2}O
(the numerals above each symbol representing the sum of the valences of
the atoms of the element present). A trivalent atom will combine with
three atoms of a univalent element, as in the compound
III III
H_{3}N.
If a trivalent element combines with a divalent element, the union will
be between two atoms of the trivalent element and three of the divalent
element, since these numbers are the smallest which have equal valences.
Thus the oxide of the trivalent metal aluminium has the formula
Al_{2}O_{3}. Finally one atom of a tetravalent element such as carbon
will combine with four atoms of a univalent element, as in the compound
CH_{4}, or with two atoms of a divalent element, as in the compound
CO_{2}.
We have no knowledge as to why elements differ in their combining power,
and there is no way to determine their valences save by experiment.
~Valence and the structure of compounds.~ Compounds will be met
from time to time which are apparent exceptions to the general
statements just made in regard to valence. Thus, from the
formula for hydrogen dioxide (H_{2}O_{2}), it might be
supposed that the oxygen is univalent; yet it is certainly
divalent in water (H_{2}O). That it may also be divalent in
H_{2}O_{2} may be made clear as follows: The unit valence of
each element may be represented graphically by a line attached
to its symbol. Univalent hydrogen and divalent oxygen will then
have the symbols H- and -O-. When atoms combine, each unit
valence of one atom combines with a unit valence of another
atom. Thus the composition of water may be expressed by the
formula H-O-H, which is meant to show that each of the unit
valences of oxygen is satisfied with the unit valence of a
single hydrogen atom.
The chemical conduct of hydrogen dioxide leads to the
conclusion that the two oxygen atoms of its molecule are in
direct combination with each other, and in addition each is in
combination with a hydrogen atom. This may be expressed by the
formula H-O-O-H. The oxygen in the compound is therefore
divalent, just as it is in water. It will thus be seen that the
structure of a compound must be known before the valences of
the atoms making up the compound can be definitely decided
upon.
Such formulas as H-O-H and H-O-O-H are known as _structural
formulas_, because they are intended to show what is known in
regard to the arrangement of the atoms in the molecules.
~Valence and the replacing power of atoms.~ Just as elements having the
same valence combine with each other atom for atom, so if they replace
each other in a chemical reaction they will do so in the same ratio.
This is seen in the following equations, in which a univalent hydrogen
atom is replaced by a univalent sodium atom:
NaOH + HCl = NaCl + H_{2}O.
2NaOH + H_{2}SO_{4} = Na_{2}SO_{4} + 2H_{2}O.
Na + H_{2}O = NaOH + H.
Similarly, one atom of divalent calcium will replace two atoms of
univalent hydrogen or one of divalent zinc:
Ca(OH)_{2} + 2 HCl = CaCl_{2} + 2H_{2}O.
CaCl_{2} + ZnSO_{4} = CaSO_{4} + ZnCl_{2}.
In like manner, one atom of a trivalent element will replace three of a
univalent element, or two atoms will replace three atoms of a divalent
element.
~Valence and its applications to formulas of salts.~ While the true nature
of valence is not understood and many questions connected with the
subject remain unanswered, yet many of the main facts are of much help
to the student. Thus the formula of a salt, differs from that of the
acid from which it is derived in that the hydrogen of the acid has been
replaced by a metal. If, then, it is known that a given metal forms a
normal salt with a certain acid, the formula of the salt can at once be
determined if the valence of the metal is known. Since sodium is
univalent, the sodium salts of the acids HCl and H_{2}SO_{4} will be
respectively NaCl and Na_{2}SO_{4}. One atom of divalent zinc will
replace 2 hydrogen atoms, so that the corresponding zinc salts will be
ZnCl_{2} and ZnSO_{4}.
The formula for aluminium sulphate is somewhat more difficult to
determine. Aluminium is trivalent, and the simplest ratio in which the
aluminium atom can replace the hydrogen in sulphuric acid is 2 atoms of
aluminium (6 valences) to 3 molecules of sulphuric acid (6 hydrogen
atoms). The formula of the sulphate will then be Al_{2}(SO_{4})_{3}.
~Valence and its application to equation writing.~ It will be readily seen
that a knowledge of valence is also of very great assistance in writing
the equations for reactions of double decomposition. Thus, in the
general reaction between an acid and a base, the essential action is
between the univalent hydrogen ion and the univalent hydroxyl ion. The
base and the acid must always be taken in such proportions as to secure
an equal number of each of these ions. Thus, in the reaction between
ferric hydroxide (Fe(OH)_{3}) and sulphuric acid (H_{2}SO_{4}), it will
be necessary to take 2 molecules of the former and 3 of the latter in
order to have an equal number of the two ions, namely, 6. The equation
will then be
2Fe(OH)_{3} + 3H_{2}SO_{4} = Fe_{2}(SO_{4})_{3} + 6H_{2}O.
Under certain conditions the salts Al_{2}(SO_{4})_{3} and CaCl_{2}
undergo double decomposition, the two metals, aluminium and calcium,
exchanging places. The simplest ratio of exchange in this case is 2
atoms of aluminium (6 valences) and 3 atoms of calcium (6 valences).
The reaction will therefore take place between 1 molecule of
Al_{2}(SO_{4})_{3} and 3 of CaCl_{2}, and the equation is as follows:
Al_{2}(SO_{4})_{3} + 3 CaCl_{2} = 3CaSO_{4} + 2AlCl_{3}.
EXERCISES
1. Sodium, calcium, and aluminium have valences of 1, 2, and 3
respectively; write the formulas of their chlorides, sulphates, and
phosphates (phosphoric acid = H_{3}PO_{4}), on the supposition that they
form salts having the normal composition.
2. Iron forms one series of salts in which it has a valence of 2, and
another series in which it has a valence of 3; write the formulas for
the two chlorides of iron, also for the two sulphates, on the
supposition that these have the normal composition.
3. Write the equation representing the neutralization of each of the
following bases by each of the acids whose formulas are given:
NaOH HCl
Ba(OH)_{2} H_{2}SO_{4}
Al(OH)_{3} H_{3}PO_{4}
4. Silver acts as a univalent element and calcium as a divalent element
in the formation of their respective nitrates and chlorides. (a) Write
the formula for silver nitrate; for calcium chloride. (b) When
solutions of these two salts are mixed, the two metals, silver and
calcium, exchange places; write the equation for the reaction.
_5._ Antimony acts as a trivalent element in the formation of a
chloride. (a) What is the formula for antimony chloride? (b) When
hydrosulphuric acid (H_{2}S) is passed into a solution of this chloride
the hydrogen and antimony exchange places; write the equation for the
reaction.
6. Lead has a valence of 2 and iron of 3 in the compounds known
respectively as lead nitrate and ferric sulphate. (a) Write the
formulas for these two compounds. (b) When their solutions are mixed
the two metals exchange places; write the equation for the reaction.