Octet rule incomplete

There are a few notable exceptions to the octet rule that you should be aware of molecules with odd numbers of electrons, incomplete octets, and expanded octets. 

Lewis Covalent and Ionic Bonds - Lewis Structures - Octet Rule -Cations and Anions - Lone Pairs - Incomplete Octets - Expanded Octets - Double and Triple Bonds - Oxyacids - Resonance. 

As mentioned earlier, the octet rule applies mainly to the second-period elements. Exceptions to the octet rule fall into three categories characterized by an incomplete octet, an odd number of electrons, or more than eight valence electrons around the central atom. 

Comment Althongh the octet rule is satisfied for the I atoms, there are only six valence electrons aronnd the A1 atom. This molecnle is an example of the incomplete octet. 

Give an example of an ion or molecule containing A1 that (a) obeys the octet rule, (b) has an expanded octet, and (c) has an incomplete octet. 

The octet rule is remarkable in its ability to realistically model bonding and structure in covalent compounds. But, like any model, it does not adequately describe all systems. Beryllium, boron, and aluminum, in particular, tend to form compounds in which they are surrounded by fewer than eight electrons. This situation is termed an incomplete octet. Other molecules, such as nitric oxide ... 

It is apparent that there is no way to satisfy the octet rule for Be in this compound. Consequently, BeHj is an exception to the octet rule. It contains an incomplete octet. 

Aromatics were thought at one time to be unsaturated because the structure was thought to have double bonds. (See the first-theory benzene structure in Figure 5.38.) The structure appeared to have three double bonds to satisfy the octet rule of bonding. However, in reality, aromatics do not behave like unsaturated compounds. They bum with incomplete combustion. They are unreactive, so it is theorized that instead of three double bonds, they have a unique structure where the six extra electrons are in a state of resonance within the benzene ring. They are not attached to any one of the carbons, but rather go from one to another at a speed faster than the speed of light, much the same way a rotor works inside a distributor in an automobile. The... 

The BCI3 molecule has an incomplete octet around B. Draw three resonance structures of the molecule in which the octet rule is satisfied for both the B and the Cl atoms. Show formal charges. 

Many chemical models are known to be incorrect, for example, the Lewis theory of bonding, or an incomplete description of chemical phenomena, for example, the octet rule (Chapter 4). For the purposes of teaching chemistry it is often preferable to use simple approximate bonding models that give correct predictions for the majority of cases, than to use a more accurate but complicated model, such as the quantum mechanical model (Chapter 12), which is based on the electron s wave properties. 

There are a number of general exceptions to the octet rule electron deficient compounds in which the central atom has an incomplete octet, free radicals (with an unpaired electron), molecules whose central atom is surrounded by more than eight electrons (an expanded octet) and compounds of hydrogen and the transition metals (d-block) and lanthanoids (f-block). 

Exceptions to the Octet Rule Odd-Electron Species, Incomplete Octets, and Expanded Octets 406... 

The octet rule in the Lewis model has some exceptions, which we examine in this section of the chapter. They include (1) odd-electron species, molecules or ions with an odd number of electrons (2) incomplete octets, molecnles or ions with fewer than eight electrons around an atom and (3) expanded octets, molecnles or ions with more than eight electrons aronnd an atom. 

Another significant exception to the octet rule involves those elements that tend to form incomplete octets. The most important of these is boron, which forms compounds with only six electrons around B, rather than eight. For example, BF3 and BFI3 lack an octet for B. 

Other exceptions to the octet rule include molecules with incomplete octets—usually totaling 6 electrons (especially important in compounds containing boron)—and molecules with expanded octets—usually 10 or 12 electrons (which can occur in compounds containing elements from the third row of the periodic table and below). Expanded octets never occur in second-period elements. 

In his landmark paper [6], Gilbert N. Lewis described the valence electron shell of an atom as a cube, so each of its vertices could be occupied by one of the eight electrons required by the octet mle. For atoms with incomplete shells, the octet rule could be achieved by sharing an electron pair with a neighboring atom, represented in his model as two cubes sharing an edge or a face (Fig. 1). 

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. 

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