Polyatomic molecules

Key points: Each Ϭ bond in a polyatomic molecule is formed by the spin pairing of electrons in any neighbouring atomic orbitals with cylindrical symmetry about the relevant internuclear axis; π bonds are formed by pairing electrons that occupy neighbouring atomic orbitals of the appropriate symmetry.

To introduce polyatomic molecules we consider the VB description of H₂O. The valence electron configuration of a hydrogen atom is 1s1 and that of an O atom is 2s²2p_z²2p_y¹2p_x¹. The two unpaired electrons in the O₂p orbitals can each pair with an electron in an H1s orbital, and each combination results in the formation of a bond (each bond has cylindrical symmetry about the respective O-H internuclear axis). Because the 2py and 2pz orbit als lie at 90° to each other, the two bonds also lie at 90° to each other (Fig. 2.6). We can predict, therefore, that H₂ O should be an angular molecule, which it is. However, the theory predicts a bond angle of 90º whereas the actual bond angle is 104.5°. Similarly, to predict the structure of an ammonia molecule, NH₃, we start by noting that the valence electron configuration of an N atom given previously suggests that three H atoms can form bonds by spin pairing with the electrons in the three half-filled 2p orbitals. The latter are perpendicular to each other, so we predict a trigonal-pyramidal molecule with a bond angle of 90°. An NH3 molecule is indeed trigonal pyramidal, but the experimental bond angle is 107º. Another deficiency of the VB theory presented so far is its inability to account for the tetravalence of carbon, its ability to form four bonds. The ground-state configuration of C is 2s²2p_z¹2p_y¹, which suggests that a C atom should be capable of forming only two bonds, not four. Clearly, something is missing from the VB approach. These two deficiencies—the failure to account for bond angles and the valence of carbon—are overcome by introducing two new features, promotion and hybridization.

Fig. 2.6 The VB description of H₂O. There are two Ϭ bonds formed by pairing electrons in O2p and H1s orbitals. This model predicts a bond angle of 90°.

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مواضيع ذات صلة

Homonuclear diatomic molecules

The hydrogen molecule

The basic shapes

Resonance

Polarizability

Electronegativity

Electron affinity

Atomic and ionic radii

The format of the periodic table

The periodic table

Penetration and shielding

The angular variation of atomic orbitals