Atomic properties valence configurations

All the elements in a main group have in common a characteristic valence electron configuration. The electron configuration controls the valence of the element (the number of bonds that it can form) and affects its chemical and physical properties. Five atomic properties are principally responsible for the characteristic properties of each element atomic radius, ionization energy, electron affinity, electronegativity, and polarizability. All five properties are related to trends in the effective nuclear charge experienced by the valence electrons and their distance from the nucleus. 

The charge on the nucleus and the number of electrons in the valence shell determine the chemical properties of the atom. The electron configurations of the noble gases (except for that of helium) correspond to a valence shell containing eight electrons—a very stable configuration called an... 

In neutral molecules and complex anions, the metal atom usually has a positive oxidation state. It therefore has a partial positive charge and a higher Zeff than that of the neutral atom. As a result, the 3d orbitals are again lower in energy than the 4s orbital, and so all the metal s valence electrons occupy the d orbitals. The metal atom in both VCI4 and MnC>42-, for example, has the valence configuration 3d1. Electron configurations and other properties for atoms and common ions of first-series transition elements are summarized in Table 20.1. 

Thus, the main relativistic effects are (1) the radical contraction and energetic stabilization of the s and p orbitals which in turn induce the radial expansion and energetic destabilization of the outer d and f orbitals, and (2) the well-known spin-orbit splitting. These effects will be pronounced upon going from As to Sb to Bi. Associated with effect (1), it is interesting to note that the Bi atom has a tendency to form compounds in which Bi is trivalent with the 6s 6p valence configuration. For this tendency of the 6s electron pair to remain formally unoxidized in bismuth compounds (i.e. core-like nature of the 6s electrons), the term inert pair effect or nonhybridization effect has been often used for a reasonable explanation. In this context, the relatively inert 4s pair of the As atom (compared with the 5s pair of Sb) may be ascribed to the stabilization due to the d-block contraction , rather than effect (1) . On the other hand, effect (2) plays an important role in the electronic and spectroscopic properties of atoms and molecules especially in the open-shell states. It not only splits the electronic states but also mixes the states which would not mix in the absence of spin-orbit interaction. As an example, it was calculated that even the ground state ( 2 " ) of Bij is 25% contaminated by Hg. In the Pauli Hamiltonian approximation there is one more relativistic effect called the Dawin term. This will tend to counteract partially the mass-velocity effect. 

Elements in the same group of the periodic table possess the same number of electrons in their outer shells and are therefore said to have the same valence electronic configuration, and consequently similar chemical and physical properties. As electrons are filled into the inner shells of an atom, the outer shell takes on a specific valence configuration that is determined by the rules that govern how many electrons can occupy a particular shell, as described above in the section on quantum mechanics. It is this very regularity in the number of outer-shell electrons that explains the periodic behavior shown by the elements as the atomic number increases. Similarly, properties such as atomic size are determined by the number of shells an atom contains. For example, the radius of the atoms of the elements within a particular group in the periodic table increases from the top of the group to the bottom. 

It is, of course, true that most properties of the elements are determined by the outer-electron configurations of their atoms. These configurations are, of course, the basis for the arrangement of elements in the periodic table. However, this effect of the valence electrons is often quite indirect. Along with properties of single atoms, properties exist that are collectively determined, for example, by the lattice structure. 

Is 2s 2p 3s 3p 3d 4s. If the 3d states were truly core states, then one might expect copper to resemble potassium as its atomic configuration is ls 2s 2p 3s 3p 4s The strong differences between copper and potassium in temis of their chemical properties suggest that the 3d states interact strongly with the valence electrons. This is reflected in the energy band structure of copper (figure Al.3.27). 

A is a parameter that can be varied to give the correct amount of ionic character. Another way to view the valence bond picture is that the incorporation of ionic character corrects the overemphasis that the valence bond treatment places on electron correlation. The molecular orbital wavefimction underestimates electron correlation and requires methods such as configuration interaction to correct for it. Although the presence of ionic structures in species such as H2 appears coimterintuitive to many chemists, such species are widely used to explain certain other phenomena such as the ortho/para or meta directing properties of substituted benzene compounds imder electrophilic attack. Moverover, it has been shown that the ionic structures correspond to the deformation of the atomic orbitals when daey are involved in chemical bonds. 

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