Valence Bond increased

The characteristic feature of valence bond theory is that it pictures a covalent bond between two atoms in terms of an m phase overlap of a half filled orbital of one atom with a half filled orbital of the other illustrated for the case of H2 m Figure 2 3 Two hydrogen atoms each containing an electron m a Is orbital combine so that their orbitals overlap to give a new orbital associated with both of them In phase orbital overlap (con structive interference) increases the probability of finding an electron m the region between the two nuclei where it feels the attractive force of both of them... 

In valence bond terms the pyrazine ring may be represented as a resonance hybrid of a number of canonical structures (e.g. 1-4), with charge separated structures such as (3) contributing significantly, as evidenced by the polar character of the C=N bond in a number of reactions. The fusion of one or two benzene rings in quinoxaline (5) and phenazine (6) clearly increases the number of resonance structures which are available to these systems. 

The unpaired electron in NO2 appears to be more localized on the N atom than it is in NO and this may explain the ready dimerization. NO2 is also readily ionized either by loss of an electron (9.91 eV) to give the nitryl cation N02 (iso electronic with CO2) or by gain of an electron to give the nitrite ion NO2 (isoeleelronic with O3). These changes are accompanied by a dramatic diminution in bond angle and an increase in N-O distance as the number of valence electrons increases from 16 to 18 (top diagram). 

In the case of grinding, the cellulose fibers go over a state of fine fibrillation into a more or less powdery substance. This mechanical severance of cellulose may break main valence bonds and will, therefore, decrease its degree of polymerization. In addition, the crystal structure of cellulose fibers is nearly lost [32]. Grinding of the cellulose fibers also, appreciably increases its surface area. 

It is of interest to note that in order for a given atom to increase its ligancy beyond its covalency it is not necessary that this atom have an extra orbital, it is instead sufficient for the atoms that surround it to have extra orbitals. The valence bonds of the central atom may then resonate among their alternative positions by pivoting about the central atom. 

Mercury, with interatomic distances 2-999(6) and 3-463(6), appears to have valency 3 . With bond numbers and, respectively, these distances lead to Rx = 1-410 and 1-498, the latter being much too large fort = 4(1 = 1-403), whereas bond numbers and lead to I i = 1-410 and 1-408, in approximate agreement with the value 1-418 for v — 3 . The decrease in valency from cadmium to mercury conforms to a general trend toward smaller metallic valencies with increasing atomic number in a group of elements. 

Phosphorus and arsenic have nearly identical electronegativities, so in GaP Asi. , the dominant effect is the smaller atomic radius of P relative to As. Substituting P atoms for As atoms shrinks the dimensions of the semiconductor lattice. This leads to greater overlap of the valence orbitals, increased stability of the bonding orbitals (valence band), and an increased band gap. 

In view of the fact that complete methylation of F N- HX to give (CH3)3N- -HX leads to an increased extent of proton transfer from HX to the base when X is Cl and essentially complete transfer when X is I, it seemed reasonable to seek a more significant contribution from the ionic valence bond structure [(CH3)3NC1] + - F in (CT N- ClF by examining properties similarly derived from its rotational spectrum [68]. 

Although the complexity increases rapidly there is no reason that Walsh diagrams cannot be constructed for XY3 pyramidal, XY4 tetrahedral, XYS octahedral, and other molecules. In fact, they have been prepared, but their applications will not be described here. Insofar as these diagrams are amenable to quantitative interpretation, the predictions are in accord with what we know from experimental evidence and valence bond methods. 

---