Ordinary valency

The valency (T) of an atom may be defined as the number of hydrogen or fluorine atoms with which the atom will combine. The assumption is made that hydrogen and fluorine are always univalent. 

To obtain the number of hydrogen or fluorine atoms (X) with which an atom (A) will combine, one looks at compounds formed between A and X, and picks out the one in which there are no A-A bonds. If this is molecular it will have the formula AX , if non-molecular (AX )co. The valency of A is then given by n. 

When neither hydrogen nor fluorine is suitable for the determination of valency, other atoms can be used, provided that their valencies have first been established relative to hydrogen or fluorine. The most useful is the oxygen atom, though this must be employed with care, for the following reasons  

The valencies of some of the more important elements are given in the Table at the end of this chapter. For some elements there is only one valency for others there are several. These valencies enable bond formulae to be drawn for many molecules of the elements concerned, as is done routinely in organic chemistry. 

Formula (II) is accepted because sulfur dioxide does not behave like a peroxide, and microwave spectroscopy gives the O-S-0 angle as 119.5° and the S-0 distance as 1.43 A. The latter is similar to the distance in molecules that have to be written with a double SO bond, e.g. 1.45 A in SOCI2. Three-membered rings, with angles of 60°, are unusual in molecules. 

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To answer Staudinger s critics, Carothers knew he had to resolve three issues. First, he would have to build one of Staudinger s macromolecules. Next, he would have to confirm that they were indeed long-chained molecules, not merely aggregates of smaller molecules, as others claimed. Finally, he would have to prove that the forces holding the chains together were ordinary valence bonds, rather than a mysterious weak force. 

Carothers had already settled the controversy between Staudinger and his opponents. He had built Staudinger s macromolecules, and the properties of his macromolecules were identical to those of natural polymers. Even more, his path breaking technique of condensation polymerization had demonstrated that his macromolecules consisted of long chains held together by ordinary valence bonds. 

But the discovery of the carboranes in the early 1960s revealed that bonding possibilities other than simple o-- or 7r-bonds between B and C centers were necessary to understand the structure of such compounds as 1,5-dicarba-closo-pentaborane(5) [6 in Eq. (1)], which is obtained in low yield in an electrical discharge.6 Ordinary valence conventions cannot account for the bonding of boron to five other atoms, and hence the concept of electron-deficient bonding must be invoked for boron. Although carbon seems to adhere to normal tetravalence, again it should be remembered... 

Such highly ionized species have been detected for Cl-37 produced by the EC decay of Ar-37 in gaseous phase ((>). In solids, however, such anomalous states are not realized or their life time is much shorter than the half-life of the Mossbauer level (Fe-57 98 ns and Sn-119 17-8 ns) because of fast electron transfer, and usually species in ordinary valence states (2+, 3+ for Fe-57 and 2+, 4+ for Sn-119) are observed in emission Mossbauer spectra (7,8). The distribution of Fe-57 and Sn-119 between the two valence states depends on the physical and chemical environments of the decaying atom in a very complicated way, and detection of the counterparts of the redox reaction is generally very difficult. The recoil energy associated with the EC decays of Co-57 and Sb-119 is estimated to be insufficient to induce displacement of the atom in solids. 

Chromium has a maximum co-ordination number of six the chromium atom, therefore, may combine with, at most, six monovalent atoms or groups, over and above its ordinary valency value, with formation of a complex radicle. Hence chromic chloride is capable of associating with, or adding on, six molecules of ammonia with formation of the derivative, [Cr(NH3)8]Cl3. Ammonia may be replaced by a substituted ammonia group or some other basic group, such as alkyl amine, pyridine, or ethylenediamine. 

This consideration ignores the difference between the ordinary valence and the metallic valence described in this chapter. For CutZn, for example, the valences 5.56 for copper and 4.56 for zinc given in Table 11-1 lead to 64.28 valence electrons per 13 atoms, and the same ratio is obtained also for CugGa, CunSn8, etc. 

Langmuir put forward an extremely definite form of this idea. The adsorbed molecules are supposed to be held to the surface by ordinary valency forces , either primary valencies or secondary valencies . In the light of recent developments in the theory of atomic structure it would probably be sufficient to say that the adsorbed molecules are attached to the molecules constituting the surface by non-polar linkages. Thus the kind of union between tungsten and oxygen adsorbed on its surface, to... 

LATTICE COMPOUNDS. Chemical compounds formed between deh-nile sloichioinctric amounts of two molecular species that owe their stability to packing in the crystal lattice, and not to ordinary valence forces. 

The method of McConnell [13] assumes the presence of a set of virtual orbitals all with higher energy than the electron to be transferred, in the form of Hiickel combinations of C 3d orbitals. This somewhat odd assumption leads to exponential decrease. Incorrectly one gets the impression that only unoccupied MO s are useful for ET. The virtual orbitals of the kind used by McConnell, are nowadays standard in any reasonable basis set for ab initio calculations, but it is fair to say that the electronic factor is not much dependent on whether they are included or not. Ordinary valence MO s of the bridge, occupied or unoccupied are of a much greater importance for ET. The McConnell method [13] is therefore mainly of historical interest. 

It is obvious that the reaction of the peroxide with an acid is a metathesis in which the 02 radical is concerned just as the O is concerned in the neutralization of an ordinary oxide. It is furthermore obvious from the formulas H202 and Na202 and Ba02, if we ascribe the ordinary valence to hydrogen, sodium, and barium, that the valence of the 02 radical is 2. 

The remaining molecules in our list are NO, 02, N2, CO, HC1, and HBr. The first of these, NO, is the most peculiar compound in the list and one of the most peculiar of the known compounds. We note that nitrogen supplies five, and oxygen six, outer electrons to the compound, making a total of eleven, an odd number. It is quite obvious that an odd number of electrons cannot form closed shells, electron pairs, or anything else associated with stable molecules. As a matter of fact, out of all the enormous number of known chemical compounds, only a handful have an odd number of electrons, and NO is almost the only well-known one of these. We shall make no effort to explain it in terms of ordinary valence theory, for it is in every way an exception, though it can be understood in terms of atomic theory. 

That the three ordinary valencies of phosphorus in compounds of the type POX3 or PQR3 do not act in one plane, but are distributed in space symmetrically with respect to one another, was demonstrated by Caven,1 who replaced chlorine atoms in the trichloride one at a time taut in different succession by various groups such as RNH— or RO—, forming, for example, the anilino-, p-toluidino- and then the P toluidino-anilino chloride. 

If we admit the hypothesis 2 that the three quantum orbits of the second series may contain a maximum of 6, 6 instead of 4, 4 electrons, it follows that PC15 also, and other quinquevalent compounds, may be written with ordinary valencies or duplet bonds only, giving shells of 10 . If, however, 8 is the maximum possible (failing the completion of 12), then PC16 must be constituted either as NII4C1, or it must contribute two electrons to a pair of chlorine atoms, thus developing a mixed bond. In the first case... 

The important emitters of firework flames are molecules with the exception of Na atoms. The molecules are produced in quite different forms from the original colour producing materials mixed into the composition. The chemical combination of the emitters are relatively simple and in general are outside the ordinary valency law. For example, they are written as SrCl, BaCl, CuCl etc. and not SrCl2i BaClj, CuCl etc. 

The vast majority of intermetallic, compounds, however, do not obey the ordinary valency laws, and have such formula as AgMg, C1u5Si, (1u5Zn8, Na31Pb8, CuZn3, etc. While it is only within recent years that Hume-Bothery and others have been able to offer rational explanations of the formulae, and, indeed, structures, of these extraordinary-valency compounds, it might be mentioned that we should not expect compounds, which are metallic in structure and properties, to have formulae which suggest that the valency electrons of the atoms concerned are completely tied in chemical combination, and are therefore unable to assist in, for example, the conduction of electricity. 

In writing the structures of addition compounds, Werner indicates auxiliary valence by dotted lines and ordinary valence by full lines. This is illustrated by his structure... 

Calcium chloride may combine with six, four, or two, molecules of water of hydration or exist in the anhydrous state. The co-ordination number of the calcium in the hexahydrate is six, but what it is in the tetrahydrate is difficult to say, since it is not known whether the chlorine is in the inner or outer zone. In the di-hydrate it may be four, and in the zero hydrate it can only be two. The external physical conditions such as concentration, vapor pressure, and temperature, determine the number of molecules of water of hydration of the calcium chloride. This shows that the co-ordination number of the calcium is dependent upon external conditions just as the ordinary valence is. 

During the controversy over the question whether polymeric substances, which show colloidal properties in solution, are to be considered as molecules or as particles built up from many small molecules, the question was raised whether soap solutions might not serve as the model for polymeric colloids. It was in fact known that soap solutions owe their colloidal properties to a reversible association of the fatty acid anions or molecules. It became however ever more clear that the polymeric colloids must be considered as macromolecules — that is to say many monomers are bound together by ordinary valencies to form a polymer. It can be deduced from many properties of the polymers that these are indeed macromolecules. 

Electrovalencies can take positive or negative values, whereas ordinary valencies can only be positive. Some elements have just one electrovalency others have more than one. As with ordinary valencies, electrovalencies enable the formulae of compounds to be worked out, and thus constitute both an aid to the memory and a tool for prediction. 

The covalencies of some important elements are given in the Table at the end of the chapter. These can be used in the same way as ordinary valencies to predict the formulae of low-polarity compounds, as discussed in the next section. 

Electrovalency is applicable to high-polarity compounds, covalency to low-polarity. Ordinary valency is applicable to all kinds. 

The equation in this form divides ordinary valency into covalency and electrovalency. 

Lewis s theory explains the relations between ordinary valency, electrovalency, and covalency found in Chapter 8 (e.g. V= e = C = 1 for hydrogen). 

An atom exhibits two types of valency, its ordinary valency (V), and a valency that determines the number of neighbouring atoms to which it is bound (the coordination munber ). 

An atom s ordinary valency is satisfied by other atoms or radicals its coordination number is satisfied by atoms, radicals, or molecules. 

G. N. Lewis proposed that an ordinary valency bond, such as that between carbon and hydrogen or carbon and chlorine (later called a covalence by... 

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