Linear triatomic molecules and sp hybridization schemes

In this section, we first discuss the bonding in two linear triatomic molecules BeH2 with only a bonds and C02 with both a and n bonds. Then we go on to treat other polyatomic molecules with the hybridization theory. Next we discuss the derivation of a self-consistent set of covalent radii for the atoms. Finally, we study the bonding and reactivity of conjugated polyenes by applying Hiickel molecular orbital theory. 

The molecular orbitals of this molecule are formed by the 2s and 2p orbitals of Be and the Is orbitals of Ha and H. Here we take the molecular axis in BeH2 as the z axis, as shown in Fig. 3.4.1. 

To form the molecular orbitals for polyatomic molecules AX , we first carry out linear combinations of the orbitals on X and then match them, taking into account their symmetry characteristics, with the atomic orbitals on the central atom A. 

For our simple example of BeH2, the valence orbitals on Ha and H, lsa and Is, can form only two linear (and independent) combinations lsa + ls and lsa - Is. We can see that combination 1 sa + 1 s matches in symmetry with the Be 2s orbital. Hence they can form both bonding and antibonding molecular orbitals  

The relative magnitudes of coefficients c and c2, as well as those of c and c 2, are determined by the relative electronegatives of the atoms concerned, which are reflected by the relative energies of the atomic orbitals. 

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