Nitrogen valence shell accommodation

The octet rule accounts for the valences of many of the elements and the structures of many compounds. Carbon, nitrogen, oxygen, and fluorine obey the octet rule rigorously, provided there are enough electrons to go around. However, some compounds have an odd number of electrons. In addition, an atom of phosphorus, sulfur, chlorine, or another nonmetal in Period 3 and subsequent periods can accommodate more than eight electrons in its valence shell. The following two sections show how to recognize exceptions to the octet rule. 

Five-electron three-centre bonding units involve the distribution of five electrons amongst three overlapping AOs that are located on three atomic centres. The 7t-electrons of triatomic systems with 19 valence-shell electrons provide examples of these types of bonding units when only valence-shell pjt AOs are used to accommodate the electrons. Another example is provided by the indirect interactions of two nitrogen atoms via a hydrogen atom in medium-ring bicyclic compounds [23]. We consider here symmetrical systems, for which there are three canonical Lewis structures, XXVII-XXIX,... 

Example 2.12. Nitrogen cannot accommodate more than eight electrons in the valence shell. 

From the foregoing discussion it appears that as a good approximation the valence shell of Ng consists of 12 molecular orbitals that can be constructed from three 2s and nine 2p orbitals of the nitrogen atoms it accommodates 16 valence electrons. 

The charge on each is zero. There is a total of eight electrons around each nitrogen in the valence shell, which normally has five accommodated there, and each nitrogen atom has a half share in the six electrons that are involved in the three bonds between the atoms. This results in a zero net charge on each atom. 

First, a neutral nitrogen atom, which has five electrons accommodated in its valence shell, and hence an oxidation number of zero, gains a single electron to give it six electrons accommodated in three lone pairs. Thus, it now has an oxidation number of-1. 

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. 

Period 2 elements can accommodate only 8 valence electrons hence the octet rule. Atoms in periods beyond 2 can hold more than 8 electrons in their valence shell. Phosphorus is a period 3 element can accommodate more than 8 electrons in its valence shell, which allows it to form more than 3 bonds. Nitrogen (a period 2 element) has a capacity for only 8 electrons in its outer shell, limiting it to 3 bonds and an unshared pair of electrons, or 4 bonds and a positive charge. 

In writing Lewis structures for molecules and ions, you must remember that elements of the second period, including carbon, nitrogen, and oxygen, can accommodate no more than eight electrons in the four orbitals (2r, 2p, 2py, and 2p ) of their valence shells. Following are two Lewis structures for nitric acid, HNO3, each with the correct number of valence electrons, namely, 24 one structure is acceptable and the other is not ... 

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