Central atoms, expanded valence level

Central Atoms with an Expanded Valence Level... 

Keep in mind that the need for an expanded valence level for the central atom may not always be as obvious as in the previous Sample Problem. For example, what if you were asked to predict the molecular shape of the polyatomic ion, BrCU" Drawing the Lewis structure enables you to determine that the central atom has an expanded valence level. 

There are not enough electrons for bonding. Therefore, the central atom requires an expanded valence level. A possible Lewis structure for BrCU is ... 

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 ... 

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. 

He and H cannot serve as central atoms in a Lewis structure. Both can have no more than two valence electrons. Fluorine needs only one electron to complete its valence level, and it does not have d orbitals available to expand its valence level. Thus, it can bond to only one other atom. 10.3 All the structures obey the octet rule except c and g. 

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. 

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... 

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