Promotion

 Key point: Promotion of electrons may occur if the outcome is to achieve more or stronger bonds and a lower overall energy.

Promotion is the excitation of an electron to an orbital of higher energy in the course of bond formation. Although electron promotion requires an investment of energy, that investment is worthwhile if the energy can be more than recovered from the greater strength or number of bonds that it allows to be formed. Promotion is not a ‘real’ process in which an atom somehow becomes excited and then forms bonds: it is a contribution to the overall energy change that occurs when bonds form. In carbon, for example, the promotion of a 2s electron to a 2p orbital can be thought of as leading to the configuration 2s¹2p_z¹2p_y¹2p_x¹, with four unpaired electrons in separate orbitals. These electrons may pair with four electrons in orbitals provided by four other atoms, such as four H1s orbitals if the molecule is CH₄ , and hence form four bonds. Although energy was required to promote the electron, it is more than recovered by the atom’s ability to form four bonds in place of the two bonds of the unpromoted atom. Promotion, and the formation of four bonds, is a characteristic feature of carbon and of its congeners in Group 14 because the promotion energy is quite small: the promoted electron leaves a doubly occupied ns orbital and enters a vacant np orbital, hence significantly relieving the electron–electron repulsion it experiences in the ground state. This promotion on an electron becomes energetically less favourable as the group is descended and divalent compounds are common for tin and lead (Section 9.5).

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

Polytypism

The close packing of spheres

Lattices and unit cells

The orbitals

Bonding and antibonding orbitals

Hybridization

Hypervalence

Polyatomic molecules

Homonuclear diatomic molecules

The hydrogen molecule

The basic shapes

Resonance