Molecular orbitals hydrogen molecule, antibonding

Figure 2.7 The close approach of two hydrogen atoms, each with an electron in a Is orbital, leads to the formation of two molecular orbitals, a bonding iris molecular orbital and an antibonding (J 1s orbital. In the H2 molecule, both electrons occupy the bonding orbital, and a strong bond with energy liibond results...

Molecular orbitals are formed when two atomic orbitals overlap. As an example, consider the overlap that occurs between the s orbitals when a molecule of hydrogen forms. Overlap occurs in two ways with reinforcement in the space between the nuclei (o orbital) or with interference between the nuclei (rr orbital), illustrated in Figure 5.2. You can see that the electron density between nuclei in the o orbital would be very small and the nuclei would repel each other, thus this molecular orbital is termed antibonding. However, the electron density is largely concentrated between the nuclei in the orbital, and the nuclei are attracted to it and, therefore, together. This is the bonding molecular orbital. 

To get the molecular orbital of the hydrogen molecule, the orbital equations of the two atoms are combined. When the orbital equations are added together, the result is a bonding molecular orbital that extends over both atoms. Subtracting the orbital equations of the atoms produces an antibonding molecular orbital. This process is called the linear combination of atomic orbitals or LCAO. 

The labels on MOs indicate their symmetries (shapes), their parent atomic orbitals, and whether they are bonding or antibonding. Antibonding character is indicated by an asterisk. For the H2 molecule both MOs have a symmetry and both are constructed from hydrogen Is atomic orbitals. The molecular orbitals for H2 are therefore labeled as follows ... 

The molecular orbitals are formed by the combination of atomic orbitals and the electronic states in the constituent atoms will determine the resultant electronic state, i.e, the value of A, in the molecule. Thus in the case of the hydrogen molecule ion the electron is in the is state in the atom and tiie molecular orbital that is formed is denoted by on 5 for the bonding orbital and or 15 for the antibonding orbital. These orbitals may be represented schematically and are shown in Figure 25 where the increase and decrease... 

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