Expanding octet

As you can see, the central atoms in these molecules have expanded octets. In PC15, the phosphorus atom is surrounded by 10 valence electrons (5 shared pairs) in SF6, there are 12 valence electrons (6 shared pairs) around the sulfur atom. 

All these atoms have d orbitals available for bonding (3d, 4d, 5d). These are the orbitals in which the extra pairs of electrons are located in such species as PCI5 and SF Because there is no 2d sublevel, C, N, and O never form expanded octets. 

Sometimes, as with PCI5 and SF6, it is clear from the formula that the central atom has an expanded octet Often, however, it is by no means obvious that this is the case. At first glance, formulas such as C1F3 or XeF4 look completely straightforward. However, when you try to draw the Lewis structure it becomes clear that an expanded octet is involved. The number of electrons available after the skeleton is drawn is greater than the number required... 

In many expanded-octet molecules, one or more of the electron pairs around the central atom are unshared. Recall, for example, the Lewis structure of xenon tetrafluoride, XeF4 (Example 7.4). 

Molecular geometries for molecules with expanded octets and unshared electron pairs. The gray spheres represent terminal atoms (X), and the open ellipses represent unshared electron pairs (E). For example. AX4E represents a molecule in which the central atom is surrounded by four covalent bonds and one unshared electron pair. 

The extra electron pairs in an expanded octet are accommodated by using d orbitals. The phosphorus atom (five valence electrons) in PC15 and the sulfur atom (six valence electrons) in SF6 make use of 3d as well as 3s and 3p orbitals ... 

In each of the following polyatomic ions, the central atom has an expanded octet. Determine the number of electron pairs around the central atom and the hybridization in... 

In which of the following molecules does the sulfur have an expanded octet For those that do, write the Lewis structure. 

Expanded octet More than four electron pairs about a central atom, 173-174 and hybridization, 187 and molecular geometry, 181 Expansion, 339-340... 

Mole-mass, 55,70-72q Molecular formula A formula in which the number of atoms of each type in a molecule is indicated as a subscript after the symbol of the atom, 34,59-60 Molecular geometry The shape of a molecule, describing the relative positions of atoms, 175 193q electron pairs, 179t major features, 175-176 molecules with expanded octets, 181t molecules with unshared electron pairs, 181t... 

They found no evidence of (Z)/(E)-isomerism in the diazosulfones formed. This may be due to the lowering of the barrier of rotation about the NN double bond by the contributions of expanded octet structures such as 6.17 b. It is therefore likely that the observed diazosulfones are ( -compounds. 

A note on good practice Although expanded valence shell is the logically precise term, most chemists still use the term expanded octet. 

Examples freezing N2(g) + 3 H2(g) - 2 NH,(g). expanded valence shell A valence shell containing more than eight electrons. Also called an expanded octet. Examples the valence shells of P and S in PC1S and SFh. expansion work See work. experiment A test carried out under carefully controlled conditions. 

The structures with expanded octets have lower formal charges, (b) There is one Lewis structure that obeys the octet rule ... 

The formal charge at sulfur can be reduced to 0 by including one double bond contribution. This change gives rise to two expanded octet structures ... 

Where both interpretations were found to be inappropriate, the classic valence concept was reformulated, either by taking a different value for Ni (e.g. the idea of expanded octets) or by reclassifying certain A1 atoms as cationic constituents in particular cases. 

Hydrogen never has an octet of electrons in any of its compounds, but rather a pair (or duet, if you prefer). An example is the Lewis structure of H20 (below). In many compounds in which the central atom is from the second period or higher, there are more than eight electrons around the central atom an example of a compound with such an expanded octet is IC13 (below). Finally, in some compounds, there are less than eight electrons around the central atom one such electron deficient compound is BF3. 

In IC13 are 7 + (3 x 7) = 28 valence electrons or 14 electron pairs. An expanded octet is necessary. 

Draw Lewis structures for the following molecules and ions, and use VSEPR theory to predict the molecular shape. Indicate the examples in which the central atom has an expanded octet. 

B) To form OF there would have to be six bonds (twelve electrons around the oxygen atom). This would violate the octet rule. O does not have an empty d sublevel into which it can form expanded octets. S has an empty 3d sublevel that it uses to form six bonds in SF. ... 

The term expanded octet is commonly used to denote the expansion of a valence shell to more than eight electrons. 

Self-Test 2.10A Write the Lewis structure for xenon tetrafluoride, XeF4, and give the number of electrons in the expanded octet. 

Occupied octahedral interstice Cation surrounded octahedrally by six anions. Expanded octet Atomic core surrounded trigonal-bipyramidally, or octahedrally, by 5, or 6, electride ions, etc. 

The correct answer is (D). Te is the only one with d orbitals capable of an expanded octet. 

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