10.2.4 Molecular Orbital Theory

Question 1

Molecular Orbital Theory

Answer 1

• Lewis dot structures, VSEPR theory, and valence bond theory all rely on atomic orbitals, and consequently fail to predict some of the properties of molecules.
• Molecular orbital theory (MO theory) can be used to explain why certain molecules do not form.

Question 2

note

Answer 2

• Lewis dot structures, VSEPR theory, and valence bond theory all rely on atomic orbitals, and consequently fail to predict some of the properties of molecules. Molecular orbital theory (MO theory) provides a better picture of how bonds form.
• Adding the wave functions for the two 1s orbitals of the atoms of dihydrogen (H 2 ) in phase shows that the greatest electron probability density lies between the two nuclei. This high electron probability density is a bonding orbital ( ), and is lower in energy than the atomic orbitals.
• Adding the wave functions for the two 1s orbitals of the atoms of H 2 out of phase results in the formation of two lobes of high electron probability density on opposite sides of the molecule with a node of electron probability density between the nuclei. These two lobes are called an antibonding orbital ( *). The antibonding orbital ( *) is
  higher in energy than the individual atomic orbitals.
• The two electrons of H 2 reside in
• Molecular orbital theory (MO theory) can be used to explain why certain molecules do not form.
• For example, molecular orbital theory predicts that He 2 will not be a stable molecule. The extra energy needed to place electrons in the antibonding orbital ( *) is greater than the energy gained by placing electrons in the bonding orbital ( ). Consequently, helium exists in nature as a monatomic gas.

Question 3

Which statement about our standard definition for a chemical bond is not true?

Answer 3

In a chemical bond, the two nuclei get as close together as possible so that their electrons are completely shared.

Question 4

Look at the plot for the formation of a molecular orbital for dihelium molecule (a molecule composed of two helium atoms).

Which statement about this diagram is not correct?

Answer 4

The electrons around each nucleus are distributed evenly.

Question 5

Which statement about electrons and orbitals is not correct?

Answer 5

From now on, we will explain electron behavior in terms of molecular orbital levels, not atomic orbital levels.

Question 6

Which statement best explains the difference between the electron distributions of a bonding orbital (e.g., H2 ) and an anti-bonding orbital (e.g., He2 )?

Answer 6

In a bonding orbital situation, the electron distribution is between the nuclei, because both nuclei want to share the electrons. In an anti- bonding circumstance, the electron distribution is around each atom. There is a void between the nuclei and the nuclei repel each other.

Question 7

Look at the plot for the formation of a molecular orbital for a hydrogen molecule.

Which statement about this diagram is not correct?

Answer 7

The greatest electron density will be around the nuclei of either atom.

Question 8

The energy diagram shows the energies involved in bonds of bonding and anti-bonding orbitals. The bonding levels for two unique 1s orbitals are equal and are also shown for your reference.

Which statement about this diagram is not correct?

Answer 8

Sites A and B represent the bonding orbitals in di-hydrogen and site A represents the bonding orbital in di-helium.

Question 9

The plot shows the atomic wave functions for two atoms involved in the formation of a molecule.

Which statement best describes this graph?

Answer 9

This plot shows the two atomic wave functions for two 1s orbitals involved in the formation of the H2 molecule

Question 10

Which statement about scientific models is not correct?

Answer 10

Models have limitations that sometimes make them inexact. This significantly limits their usefulness.

Question 11

We earlier investigated the probability density of a 1s orbital and discovered that there was a greater probability for electrons to be found near the nucleus. Now we are investigating molecules. Which statement about probability densities is true?

Answer 11

A molecule is not spherical. Therefore, we describe electron density along internuclear axes, because more than one atom is involved in a molecule.

Question 12

Which statement about molecular orbitals is not true?

Answer 12

Electrons within a molecule are best described as being in atomic, not molecular, orbitals.