Effective nuclear charge molecules

See Standard oxidation voltage See Standard reduction voltage Effective nuclear charge Positive charge felt by the outermost electrons in an atom approximately equal to the atomic number minus the number of electrons in inner, complete levels, 154 Efflorescence Loss of water by a hydrate, 66 Effusion Movement of gas molecules through a pinhole or capillary,... 

A question which has been keenly argued for a number of years is the following if it were possible continuously to vary one or more of the parameters determining the nature of a system such as a molecule or a crystal, say the effective nuclear charges, then would the transition from one extreme bond type to another take place continuously, or would it show discontinuities For example, are there possible all intermediate bond types between the pure ionic bond and the pure electron-pair bond With the development of our knowledge of the nature of the chemical bond it has become evident that this question and others like it cannot be answered categorically. It is necessary to define the terms used and to indicate the point of view adopted and then it may turn out, as with this question, that no statement of universal application can be made. 

A The p values for the molecules B2, C2, N2, 02 and F2 are calculated from the internuclear distances and the Slater values for the effective nuclear charges of the respective atoms ... 

The weakness of the covalent bond in dilithium is understandable in terms of the low effective nuclear charge, which allows the 2s orbital to be very diffuse. The addition of an electron to the lithium atom is exothermic only to the extent of 59.8 kJ mol-1, which indicates the weakness of the attraction for the extra electron. By comparison, the exother-micity of electron attachment to the fluorine atom is 333 kJ mol-1. The diffuseness of the 2s orbital of lithium is indicated by the large bond length (267 pm) in the dilithium molecule. The metal exists in the form of a body-centred cubic lattice in which the radius of the lithium atoms is 152 pm again a very high value, indicative of the low cohesiveness of the metallic structure. The metallic lattice is preferred to the diatomic molecule as the more stable state of lithium. 

Johnson and Rice used an LCAO continuum orbital constructed of atomic phase-shifted coulomb functions. Such an orbital displays all of the aforementioned properties, and has only one obvious deficiency— because of large interatomic overlap, the wavefunction does not vanish at each of the nuclei of the molecule. Use of the LCAO representation of the wavefunction is equivalent to picturing the molecule as composed of individual atoms which act as independent scattering centers. However, all the overall molecular symmetry properties are accounted for, and interference effects are explicitly treated. Correlation effects appear through an assigned effective nuclear charge and corresponding quantum defects of the atomic functions. 

There are two parameters in the atomic coulomb functions, the effective nuclear charge and the quantum defect. The values of these were taken by Johnson and Rice from available spectral data. The effective atomic charge was adjusted to give the correct ionization potential of the molecule, 9.25 eV, requiring thereby z = 0.8243. The quantum defects of carbon were taken from the appropriate atomic series and were 1.04 for the 5-state and 0.73 for the p-states. It is interesting to compare the calculated molecular quantum defects (i.e., those corresponding to the Johnson-Rice LCAO function) with those which can be obtained from the various benzene Rydberg series.218 The asymptotic form of the elu orbital constructed from s atomic functions is... 

If there is a halogen atom bonded directly to the transition metal atom a relatively low specific intensity for the molecule results (2, 30, 52,109). The explanation is probably that the halogen atom increases the effective nuclear charge of the central atom and lowers the energy of the d orbitals. This removes some electron density from the orbitals of the carbonyl groups and thus decreases the specific intensity. The effect has even been claimed to be additive for some types of derivatives (109). It has been established that the specific intensity is related to the inductive character of the halogen atom and decreases in the order I > Br > Cl. In some cases, a linear relationship appears to exist (52). Extending these ideas, Nesmeyanov et al. 107)... 

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