Valence number

F. 1-26. (a) ir-Bond order of the C-S bonds in the ground state, (fc) ir-Bond order of the C-S bonds in the first excited state, (c) Free-valence number of the intermediate diradicaf. (Most probable bicyclic intermediate resulting from the ring closure of the diradicai. 

Univalent radicals have the endings -enyl, -ynyl, -dienyl, -diynyl, etc. When necessary, the positions of the double and triple bonds are indicated by locants, with the carbon atom with the free valence numbered as 1. Examples ... 

Early studies of photocyclization reactivity using Coulson s Free Valence Numbers (Ff) were carried out by Scholz, Dietz and Miihlstadt and by Laarhoven et al. Relatively good reactivity predictions for several parallel cyclization paths were obtained taking the sums of excited state Free Valence numbers for the reacting C atom pair (S F ) as reactivity measure In this sense a threshold value of 2 F = 1 was assumed, 2F < 1 implying lack of reactivity (cf also Ref. ). 

The selectivity is caused by the electron distribution in the excited diarylethylene. Several reactivity parameters for the photocyclization have appeared to be valuable. The most simple one is the sum of the free valence numbers of the atoms r and s involved in the cyclization in the excited state (E F s) (Fr = /3 — IP, P = bond order)13). The following rules could be derived from a large number of photocyclization reactions 14) ... 

It is noted that in the synthesis of 52 the precursor 53 gave the product in good yield (70 %) (only small amounts of other photocyclization products were isolated), whereas the distyryl aromatic 51 did not yield the required product, in accordance with the relevant free valence numbers. 

In Table 27 the simple Hiickel indices, Nr (Dewar number), Fr (free valence number) and Lr (localization energy) are given, together with the Mulliken overlap population pr, calculated by the extended Hiickel method and using the geometry of hexahelicene as determined by X-ray analysis141) to account for the non-planarity. 

The valence electrons are the ones you can use to build the structure. Account for any extra or missing electrons in the case of ions. For example, if you know your molecule has a -1-2 charge, remember to subtract 2 from the total number of valence electrons. If your molecule has a -2 charge, remember to add 2 to the total valence number. 

In the mathematical theory of networks valence is defined as the number of links terminating at a node, and it was in this sense that the term was introduced into chemistry. However, chemists were later forced to distinguish between a chemical valence (bonding power) and a coordinative valence (number of bonds). They chose to keep the term valence for the chemical valence and introduced the term coordination number for the coordinative valence. This book follows the chemical convention. The term valence is always used in the sense of bonding power unless otherwise stated, and coordination number is used to indicate the number of bonds. 

Matsunaga (15) applied the magnetic techniques of Eischens and Selwood and the chemisorption and chemical techniques of Voltz and Weller to a series of chromia-alumina catalysts. He found that in the limit of low chromia contents, where Eischens and Selwood deduced a two-dimensional distribution of chromium ions, treatment with oxygen at 450°C. resulted in an average valence number of six for all of the chromium ions in the sample. 

Coordination Number. The valence number of the central atom of addition compds. It indicates the number of molecules or atoms held by that atom. Carbon related elements have a max of 4, all other elements have generally a coordination number of 6. Thus [PtXg] R2 or [CrXg] R2, where R is usually a monovalent radical, while X is Br, Cl, CN, SCN, NO, N02 NH3 or H20... 

Marszall (1988) studied the effect of electrolytes on the cloud point of mixed ionic-nonionic surfactant solutions such as SDS and Triton X-100. It was found that the cloud point of the mixed micellar solutions is drastically lowered by a variety of electrolytes at considerably lower concentrations than those affecting the cloud point of nonionic surfactants used alone. The results indicate that the factors affecting the cloud point phenomena of mixed surfactants at very low concentrations of ionic surfactants and electrolytes are primarily electrostatic in nature. The change in the original charge distribution of mixed micelles at a Lxed SDS-Triton X-100 ratio (one molecule per micelle), as indicated by the cloud point measurements as a function of electrolyte concentration, depends mostly on the valency number of the cations (counterions) and to some extent on the kind of the anion (co-ion) and is independent of the type of monovalent cation. 

It is important to understand that the valency number concept is based upon a molecular view of chemistry. Frankland was careful to warn his... 

It should be apparent that valency number and oxidation number are two quite different concepts, not to be confused. Where it is possible to assign either a valency or an oxidation number to an atom, the two are often the same, but this is not invariably so. For example, in the N2... 

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