Bonding between unlike atoms

When a molecular orbital, whether of or tt type, is formed between atoms of two different elements, A 

A polyatomic molecule may contain a number of polar covalent bonds. For example, water (H2O) is a polar molecule as the two O—H bonds form dipoles pointing towards the hydrogen atoms. However, not all molecules containing several dipoles are polar, as the dipoles within the molecule, the internal dipoles, may sum to zero. 

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Polar bond A chemical bond that has positive and negative ends characteristic of all bonds between unlike atoms, 182-183... 

Hydrogen bonds between unlike atoms, as in NH4F, would not lead to residual entropy. 

In a covalent single bond between unlike atoms, the electron pair forming the a bond is never shared absolutely equally between the two atoms it tends to be attracted a little more towards the more electronegative atom of the two. Thus in an alkyl chloride (20), the... 

The gas in now mixed and recondensed. The P bonds are all reestablished but now each molecule has a number of neighbours consisting of A and B and not, as before, neighbours of the same kind. For every bond formed between two like atoms, an energy LAjP or Lb/P will be released, and for bonds between unlike atoms there will be a different value which we will call LABjP. 

To test this conclusion we need values of the energies of normal covalent bonds between unlike atoms. These values might be calculated by quantum-mechanical methods it is simpler, however, to make a postulate and test it empirically. Since a normal covalent bond A—B is similar in character to the bonds A —A and B—B, we expect the value of the bond energy to be intermediate between the values for A—A and B—B result follows from the postulate of the... 

It is probable that in general the postulate of the geometric mean leads to somewhat more satisfactory values for the energy of normal covalent bonds between unlike atoms than does the postulate of additivity. The postulate of the geometric mean is more difficult to apply than the postulate of additivity, however, since values of A can be obtained directly from heats of reaction, whereas knowledge of individual bond energies is needed for the calculation of values of A, and in the following sections of this chapter wre shall sometimes use the postulate of additivity. 

It was suggested by Schomaker knd Stevenson11 in 1941 that these deviations result from the partial ionic character of the bonds between unlike atoms. They proposed that the radii for N, O, and F be taken to be those given by the N—N, O—O, and F—F distances (Table 7-5), and that in general the interatomic distance for a bond A—B be taken to be the sum of the radii for the atoms A and B with a correction term —0.09 A xa — Xb, in which xa — Xb is the absolute value of the difference in electronegativity of the two atoms. 

The consideration of interatomic distances shows that electron transfer from atoms of one element to those of another takes place in many interatomic compounds, and that the numbers of electrons involved are reasonable in relation to the changes in valence resulting from loss or gain of electrons and to the partial ionic character of the bonds between unlike atoms and the striving of atoms toward electroneu-trality. ... 

If we select any bond between unlike atoms from the table above, calling such a bond A—B, it appears that the energy of this A—B bond is almost always greater than the geometric mean of the energies of bonds A—A and B—B. Three examples are listed ... 

Neutron diffraction experiments (522) indicate that there is no localized atomic moment in ordered FeAl. This result implies that s-p bonding between unlike atoms is considerably less stable relative to the iron d levels than that between like atoms, so that the localized, iron kg orbitals are filled. Since aluminum is electropositive with... 

There are two factors at work in the formation of bonds between unlike atoms. First, it is well known that there is a polarization of the electron pair in bonds between unlike atoms. Indeed, when Pauling formulated the electronegativity scale in 1932 [60], he was motivated by evidence that, essentially without exception, single bonds between atoms A and B are... 

On this basis a numerical scale, based on thermochemical data and designed to account for the increased strength of covalent bonds between unlike atoms,... 

In discussing bonds between unlike atoms, it is convenient to associate with every atom a quantity, x, representing its electron-attracting power in a bond, such that the ionic character of a bond P—Q is determined by It is clear that the definition x cc (/p + 4p), in terms of ionisation potential and electron affinity, might be satisfactory for the ease with which P+Q and P"Q+ could be formed would depend (p, 88) on... 

These correspond to the same energy value even when A and B are not identical hence there is just the same resonance stabilizing an electron-pair bond between unlike atoms as between like atoms. Moreover, the influence of the ionic terms is such as to... 

Compounds. As soon as one changes from elements, where the adjacent atoms are identical and the bonds are necessarily non-polar, to compounds, there enters the vexatious question of when to describe a substance as ionic and when to describe it as covalent. No attempt will be made here to deal with this question in detail for the practical reason that, very largely, there is no need to have the answer—even granting, for the sake of argument only, that any such thing as the answer exists. Suffice it to say that bonds between unlike atoms all have some degree of polarity and (a) when the polarity is relatively small it is practical to describe the bonds as polar covalent ones and (b) when the polarity is very high it makes more sense to consider that the substance consists of an array of ions. 

Atoms of two different elements always differ at least slightly in their affinity for the electrons. Hence, covalent bonds between unlike atoms are always unsym-metrical, respectively polar (Masterton... 

When is positive, bonds between unlike atoms are preferred and greater enrichment occurs at low bulk concentrations, but if it is negative bonds between like atoms are preferred and greater enrichment occurs at high bulk concentrations. The somewhat complex equations which describe surface enrichment for real solutions, i.e. when is not zero, can also predict concentration differences in second, third and fourth layers. 

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