Lewis structure expanded valence shell

For SO4", which is isoelectronic with F3SN, the Lewis octet, expanded valence-shell and increased-valence (with no valence-shell expansion) structures are of types (4)-(6). Similar valence-bond structures for the XYSO2 type sulphones of Table 17-1, which also have 32 valence-shell electrons, are structures (34)-(36). 

EXAMPLE 2.7 Writing a Lewis structure with an expanded valence shell... 

The fluoride SF4 forms when a mixture of fluorine and nitrogen gases is passed over a film of sulfur at 275°C in the absence of oxygen and moisture. Write the Lewis structure of sulfur retrafluoride and give the number of electrons in the expanded valence shell. 

S2.3 The Lewis structures and molecular shapes for XcFi and ICb are shown below. The XeFi Lewis structure has an octet for the 4 F atoms and an expanded valence shell of 10 electrons for the Xe atom, with the 8 + (2 x 7) = 22 valence electrons provided by the three atoms. The five electron pairs around the central Xe atom will anange themselves at the comers of a trigonal bipyramid (as in PF5). The three lone pairs will be in the equatorial plane, to minimize lone pair-lone pair repulsions. The resulting shape of the molecule, shown at the right, is linear (i.e., the F-Xe-F bond angle is 180°). 

Each of the tetrahedral AX3Y molecules of this Section has 32 valence-shell electrons, as have the sulphones and AO4 anions of Sections 17-4 and 17-5. Consideration of the bonding for these AX3Y molecules provides some support for the hypothesis that fluorine atoms can stabilize Pauling 3-electron-bonds in 4-electron 3-centre bonding units for neutral molecules, and that Lewis structures with expanded valence-shells for sulphur atoms could be appropriate if the sulphur atoms acquire +2 formal charges in the Lewis octet structures. 

If we use the expanded valence-shell Lewis-structure (33) to represent the electronic structure of FSN, we cannot account for the observed lengthening of the S-F bond relative to that of a single bond. By contrast, the S-F bond-length of 1.55 A for F3SN is essentially that of a single bond, and the expanded valence-shell Lewis structure (27) is in accord with this observation. This structure for F3SN is also able to account for the observed shortening of the S-N bond (1.42 A) relative to those of free SN (1.50 A) and FSN (1.45 A), whose electronic structures are to be described by (28) and resonance between (32) and (33) respectively. 

We have consistently tried to write Lewis structures in which all atoms except H have a complete octet, that is, in which each atom has eight valence electrons. There are a few Lewis structures that break this rule by having 10 or even 12 valence electrons around the central atom, creating what is called an expanded valence shell. Describing bonding in these structures is an area of active interest among chemists. 

Molecules with expanded valence shells typically involve nonmetal atoms of the third period and beyond that are bonded to highly electronegative atoms. For example, phosphorus forms two chlorides, PCI3 and PCI5. We can write a Lewis structure for PCI3 with the octet rule. In PCI5, with five Cl atoms bonded directly to the central P atom, the outer shell of the P atom appears to have ten electrons. We might say that the valence shell has expanded to ten electrons. In the SFg molecule, the valence shell appears to expand to 12. 

Expanded valence shells have also been used in cases where they appear to give a better Lewis structure than strict adherence to the octet rule, as suggested by the two Lewis structures for the sulfate ion that follow. 

Exceptions to the Octet Rule—There are often exceptions to the octet rule. (1) Odd-electron species, such as NO, have an unpaired electron and are paramagnetic. Many of these species are reactive molecular fragments, such as OH, called free radicals. (2) A few molecules have incomplete octets in their Lewis structures, that is, not enough electrons to provide an octet for every atom. (3) Expanded valence shells occur in some compoimds of nonmetals of the third period and beyond. In these, the valence shell of the central atom must be expanded to 10 or 12 electrons in order to write a Lewis structure. 

In which of the following species is it necessary to employ an expanded valence shell to represent the Lewis structure PI3, ICI3, OSCI2... 

Draw plausible Lewis structures for the following species use expanded valence shells where necessary. (a) CI2O (b) PF3 (c) CO32- (d) BrFj. 

Phosphorus pentachloride is an ionic solid consisting of PC14+ cations and PC16 anions, and it sublimes at 160°C to a gas of PCI, molecules. The Lewis structures of the polyatomic ions and the molecule are shown in (33). Although the cation is a polyatomic ion in which the P atom does not need to expand its valence shell, in the anion the P atom has expanded its valence shell to 12 electrons, by making use of two of its 3d-orbitals. In PC15, the P atom has expanded its valence shell to 10 electrons by using one 3d-orbital. 

STRATEGY A fluorine atom forms only single bonds, so we anticipate that the Lewis structure consists of a shared pair between the central S atom and each of the four surrounding F atoms. However, each F atom has three lone pairs and supplies one bonding electron, and the S atom already has six elec-5 trons in its valence shell. So, there are two extra electrons. Because sulfur is in Period 3 and has empty 3d-orbitals available, it can expand its octet. 

A drawing of a two-dimensional, electron-domain model of a conventional Lewis lone pair is shown in Fig. 23. The lone pair and bonding pairs are structurally equivalent they have identical van der Waals envelopes. Such seems to be nearly the case for lone pairs in the valence-shells of small-core, non-octet-expanding atoms (carbon, nitrogen, oxygen and fluorine). 

One important goal when deriving Lewis structures is to associate each atom with an octet of electrons, the same number of electrons found in the valence shells of the noble gases. In reality, only a few elements consistently achieve an exact octet of electrons in covalent compounds, but those that do are the important elements found in the first and second periods of the periodic table, most notably H, C, N, O, and F. Elements in the third and higher periods have more empty orbitals (d-orbitals) in their valence shells and can expand their capacity to accommodate as many as 10, 12, or even 14 electrons. Elements like P, S, I, and several others can form compounds like PC15, SFs, and IF7. Yet, these same elements form many compounds and ions with an octet of electrons in their valence shells. Other elements, like boron (Group IIIA), have only three valence electrons, and when all are used to form bonds, as in BF3, boron ends up with only six electrons in its valence shell. 

Answer The outer-shell electron configurations for P and F are 3s 3p and 2s 2p, respectively, and so the total number of valence electrons is 5 + (5 X 7), or 40. Phosphoras, like sulfur, is a third-period element, and therefore it can have an expanded octet. The Lewis structure of PF5 is... 

A stepwise process is used to convert a molecular formula into a Lewis structure, a two-dimensional representation of a molecule (or ion) that shows the relative placement of atoms and distribution of valence electrons among bonding and lone pairs. When two or more Lewis structures can be drawn for the same relative placement of atoms, the actual structure is a hybrid of those resonance forms. Formal charges are often useful for determining the most important contributor to the hybrid. Electron-deficient molecules (central Be or B) and odd-electron species (free radicals) have less than an octet around the central atom but often attain an octet in reactions. In a molecule (or ion) with a central atom from Period 3 or higher, the atom can hold more than eight electrons by using d orbitals to expand its valence shell. 

It is also possible to encounter a molecule that seems to have too many valence electrons. When that happens, we expand the valence shell of the central atom. Consider the Lewis structure for sulfur tetrafluoride SF4, for example, which contains thirty-four valence electrons. 

The third and largest class of exceptions consists of molecules or ions in which fhere are more fhan eight electrons in fhe valence shell of an atom. When we draw the Lewis structure for PCI5, for example, we are forced to "expand" the valence shell and place 10 electrons around the central phosphorus atom. 

Exception 3. Beyond the second row, the simple Lewis model is not strictly apphed, and elements may be surrounded by more than eight valence electrons, a feature referred to as valence-shell expansion. For example, phosphorus and sulfur (as relatives of nitrogen and oxygen) are trivalent and divalent, respectively, and we can readily formulate Lewis octet structures for their derivatives. But they also form stable compounds of higher valency, among them the familiar phosphoric and sulfuric acids. Some examples of octet and expanded-octet molecules containing these elements are shown below. 

In some cases, to draw octet Lewis structures, charge separation is necessary that is, guideline 1 takes precedence over guideline 3. An example is carbon monoxide. Other examples are phosphoric and sulfuric acids, although valence-shell expansion allows the formulation of expanded octet structures (see also Section 1-4 and guideline 1). 

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