Bond Orbitals with Large p Character

In a molecule such as ammonia, NH3, with structural formula 

It was pointed out in Chapter 5 that a 2s electron is more stable than a 2p electron. The difference in energy is about 750 kJ mole . The nitrogen atom. N -, is 750 kJ mole more stable if the pair of electrons is in the 2.9 orbital 2s 2p,r2p,j2pg) than if it is in one of the 2p orbitals 2s2p/2pf,2pg). Hence it tends to retain the 2 pair in forming compounds, and to use the 2pj., 2p , and 2p, orbitals for the bonding electrons. 

The experimental values of bond angles for atoms with unshared electron pairs usually lie between 90° and 109°. For example, the spectroscopically determined value for NH3 is 107°, for H2O 104.5°, for PHa93°, for H2S 92°, and for H2Se 91°. In NH3, H2O, and SCI2, in which the Cl—S—Cl angle is 102°, part of the increase in angle above 90° is because of crowding between the two atoms attached to the central atom (see Section 6-15). 

Example 6-8. The ordinary form of sulfur (orthorhombic sulfur, yellow crystals) contains octatomic molecules, . Discuss the structure of the Ss molecule. 

Solution. The sulfur atom, with four stable outer orbitals and six outer electrons, can form single covalent bonds with two other atoms. These bonds may hold the molecule together either into a ring, such as an S ring, or into a very long chain, with the two end atoms having an abnormal structure  

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