Expanded valence energy level

Draw and compare the Lewis structures of CIO4 and OSCI2. In which of these cases, if any, does the central atom have an expanded valence energy level ... 

Expanded Valence Shells Many molecules (and ions) have more than eight valence electrons around the central atom. An atom expands its valence shell to form more bonds, which releases energy. The central atom must be large and have empty orbitals that can hold the additional pairs. Therefore, expanded valence shells (levels) occur only with nonmetals from Period 3 or higher because they have d orbitals available. Such a central atom may be bonded to more than four atoms or to four or fewer. 

Fio. 1. Orbital energy level diagram for carbon oxysulphide using theoretical values (Clementi, 1962). The deeper levels are essentially localized atomic orbitals. The energy scale is in electron volts, expanded on the right to show the valence shell structure. 

We see that sp d hybridization uses an available d orbital in the outermost occupied shell of the central atom. The heavier Group VA elements—P, As, and Sb—can form five covalent bonds using this hybridization. But nitrogen, also in Group VA, cannot form five covalent bonds, because the valence shell of N has only one t and three orbitals (and no d orbitals). The set of t and orbitals in a given energy level (and therefore any set of hybrids composed only of t and p orbitals) can accommodate a maximum of eight electrons and participate in a maximum of four covalent bonds. The same is true of all elements of the second period, because they have only t and p orbitals in their valence shells. No atoms in the first and second periods can exhibit expanded valence. 

The six S—F bonds are formed by the overlap of the hybrid orbitals of the S atom and the 2p orbitals of the F atoms. Since there are 12 electrons around the S atom, the octet rule is violated. The use of d orbitals in addition to s and p orbitals to form an expanded octet (see Section 9.9) is an example of valence-shell expansion. Second-period elements, unlike third-period elements, do not have 2d energy levels, so they can never expand their valence shells. Hence atoms of second-period elements can never be surrounded by more than eight electrons in any of their compounds. 

The problem with expanded valence-shell structures is, of course, to explain where the "extra" electrons go. This expansion has been rationalized by assuming that after the 3s and 3p subshells of the central atom fill to capacity (eight electrons), extra electrons go into the empty 3d subshell. If we assume that the energy difference between the 3p and 3d levels is not very large, the valence-shell expansion scheme seems reasonable. But is this a valid assumption The use of the 3d orbitals for valence-shell expansion is a matter of scientific dispute. Although unresolved questions about the expanded... 

Fig. 2 Valence band photoemission profiles of empty C82 and Gd C82, recorded at room temperature with He Ia (21.22 eV) radiation. The inset shows the region close to the Fermi level on an expanded scale. For this photon energy, the photoionisation cross sections of the C 2s and C 2p levels dominate that of the Gd 4f levels...

The activation energy of the carriers, calculated from the temperature dependence a = for a stoichiometric sample [15], is about 0.7 eV it thus represents twice the energy gap between the edge of the valence band and the position of the free 3dz> levels of chromium atoms. The decrease of AE on deviation from the stoichiometric state, to 0.5-0.4 eV, can be interpreted either as a change in position of the levels, or as a rise of the edge of the valence band due to the acceptor band of excess atoms expanding and merging with it when the concentration of excess atoms is increased. 

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