Rabu, 01 Juni 2011

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,
     H2O,
       H3N,
         H4C.
(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 Cu2O and CuO, CO and CO2, FeCl2 and FeCl3. 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
I II
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
ZnCl2
and H2O
(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 H3N.
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 Al2O3. Finally one atom of a tetravalent element such as carbon will combine with four atoms of a univalent element, as in the compound CH4, or with two atoms of a divalent element, as in the compound CO2.
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 (H2O2), it might be supposed that the oxygen is univalent; yet it is certainly[Pg 119] divalent in water (H2O). That it may also be divalent in H2O2 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 + H2O.
2NaOH + H2SO4 = Na2SO4 + 2H2O.
Na + H2O = 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 = CaCl2 + 2H2O.
CaCl2 + ZnSO4 = CaSO4 + ZnCl2

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 H2SO4 will be respectively NaCl and Na2SO4. One atom of divalent zinc will replace 2 hydrogen atoms, so that the corresponding zinc salts will be ZnCl2 and ZnSO4.

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 Al2(SO4)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 (H2SO4), 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 + 3H2SO4 = Fe2(SO4)3 + 6H2O. 

Under certain conditions the salts Al2(SO4)3 and CaCl2 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 Al2(SO4)3 and 3 of CaCl2, and the equation is as follows:
Al2(SO4)3 + 3 CaCl2 = 3CaSO4 + 2AlCl3.

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