Question

Molecular Orbital Theory

1.1 Which model, the Lewis electron dot structure or the MO energy level diagram, better describes the bonding in O₂? Remember that liquid O₂ is attracted into the magnetic field of a large magnet.

True or False.

_____ MO theory allows us to avoid the use of resonance.

_____ Resonance is a concept which arises from the limitations of Lewis electron dot structures

1.2 As we go from Li₂ to Li₃ to Li₄ to Li_n, the number of molecular orbitals formed from the 2s atomic orbitals goes from 2 to __ to __ to __ respectively, and the spacing between energy levels _________.

1.3 In the case of a metal, the lowest unoccupied MO is only infinitesimally higher in energy than the highest occupied MO, so it takes very little energy to cause electrons to move in a metal wire. In the case of a semiconductor, there is a _____ band gap between the full valence band and the empty conduction band. In the case of an insulator, there is a _____ band gap between the full valence band and the empty conduction band.

Answer 1

1.1. The MO energy diagram explains the paramagnetic character of O₂ molecule whereas the Lewis dot structure does not describes much. Hence, the MO energy diagram describes the bonding in O₂ better than the Lewis dot structure.

The MO theory does not considers resonance structure therefore, the statement is true.

The inability of Lewis electron dot structures led to the discovery of resonance theory, hence, the statement is true.

1.2. The number of molecular orbitals formed is equal to the number of atomic orbitals combined together to form a molecular orbital. Thus, when 3 Li atomic orbitals will combine 3 MO will be formed, if 4 Li will combine 4 MO will be formed and thus if n Li will combine n MO will be formed.

The spacing between the energy levels will decrease.

1.3. In case of semiconductors the band gap between the valence band and conduction band is less due to which the electrons in valence band can be easily excited to the conduction band hence, semiconductors can conduct electricity. Whereas there is a large gap between the valence and conduction band of insulators due to which the electrons cannot be excited from the valence band to the conduction band easily hence, the insulators are bad conductors of electricity.