CHE 202 - Ch. 10

A molecule with the formula AB(3) has a trigonal planar geometry. How many electron groups are on the central atom?

3 electron groups. A trigonal planar molecular geometry has three bonding groups and no lone pairs of electrons, so there are three electron groups on atom A. This is the only way to obtain a trigonal planar molecule.

A molecule with the formula AB(3) has a trigonal pyramidal geometry. How many electron groups are on the central atom (A)?

4 electron pair(s)

What is a bonding molecular orbital?

A bonding molecular orbital is lower in energy than the atomic orbitals from which it is formed.There is an increased electron density in the internuclear region.

Explain the difference between a paramagnetic species and a diamagnetic one.

A paramagnetic species has unpaired electrons in molecular orbitals of equal energy. A paramagnetic species is attracted to a magnetic field. The magnetic property is a direct result of the unpaired electrons. The spin and angular momentum of the electrons generate tiny magnetic fields. A diamagnetic species has all of the electrons paired. The magnetic fields caused by the electron spin and orbital angular momentum tend to cancel each other. A diamagnetic species is not attracted to a magnetic field, and is, in fact, slightly repelled.

What is an antibonding molecular orbital?

An antibonding molecular orbital is higher in energy than the atomic orbitals from which it is formed. There is less electron density in the internuclear region, which results in a node.

Write a short paragraph describing chemical bonding according to the Lewis model, valence bond theory, and molecular orbital theory. Indicate how the theories differ in their description of a chemical bond and indicate the strengths and weaknesses of each theory. Which theory is correct?

In Lewis theory, a chemical bond is the transfer or sharing of electrons represented as dots.Lewis theory allows us to predict the combination of atoms that form stable molecules, and the general shape of a molecule. Lewis theory is a quick way to predict the stability and shapes of molecules based on the number of valence electrons. However, it does not deal at all with how the bonds that we make are formed. Valence bond theory is a more advanced bonding theory that treats electrons in a quantum‐mechanical manner. A quantitative approach is extremely complicated but a qualitative approach allows an understanding of how the bonds are formed.In valence bond theory, electrons reside in quantum‐mechanical orbitals localized on individual atoms. When two atoms approach each other, the electrons and nucleus of one atom interact with the electron and nucleus of the other atom. If the energy of the system is lowered, a chemical bond forms. So, valence bond theory portrays a chemical bond as the overlap of two half‐filled atomic orbitals. The shape of the molecule can be predicted from the geometry of the overlapping orbitals. Also, valence bond theory explains the rigidity of the double bond.However, valence bond theory falls short in explaining certain phenomenon such as magnetism and certain bond properties. Valence bond theory treats the electrons as if they reside in the quantum‐mechanical orbitals that we calculate for an atom. This is an oversimplification that is partially compensated for by introducing the concept of hybridization. An even more complex quantum‐mechanical model is molecular orbital theory. In molecular orbital theory, a chemical bond occurs when the electrons in the atoms can lower their energy by occupying the molecular orbitals of the resultant molecule. The chemical bonds in MO theory are not localized between atoms, but spread throughout the entire molecule. Molecular orbital theory uses trial functions to solve the Schrödinger equation for the molecules. In order to determine how well the trial function works, you calculate the energy, trying to minimize the energy. However, no matter how "good" your guess, you can never do better than nature at minimizing energy. These minimum‐energy calculations for orbitals must be done by computer. All three of these models have strengths and weaknesses, none is "correct." What information you need, depends on which approach you use.

What is a chemical bond according to molecular orbital theory?

In molecular orbital theory, atoms will bond when the electrons in the atoms can lower their energy by occupying the molecular orbitals of the resultant molecule.

Explain the difference between hybrid atomic orbitals in valence bond theory and LCAO molecular orbitals in molecular orbital theory.

In valence bond theory, hybrid orbitals are weighted linear sums of the valence atomic orbitals of a particular atom, and the hybrid orbitals remain localized on that atom. In molecular orbital theory, the molecular orbitals are weighted linear sums of the valence atomic orbitals of all the atoms in a molecule, and many of the molecular orbitals are delocalized over the entire molecule.

How is the number of molecular orbitals approximated by a linear combination of atomic orbitals related to the number of atomic orbitals used in the approximation?

Molecular orbitals can be approximated by a linear combination of atomic orbitals (AOs). The total number of MOs formed from a particular set of AOs will always equal the number of AOs used.

In molecular orbital theory, what is nonbonding orbital?

Nonbonding orbitals are atomic orbitals not involved in a bond and will remain localized on the atom.

In molecular orbital theory, what is bond order? Why is it important?

The bond order in a diatomic molecule is the number of electrons in bonding molecular orbitals(MOs) minus the number in antibonding MOs divided by two. The higher the bond order, the stronger the bond. A negative or zero bond order indicates that a bond will not form between the atoms.

Why does the energy ordering of the molecular orbitals of period 2 diatomic molecules change in going from N (2) to O(2)?

The degree of mixing between two orbitals decreases with increasing energy difference between them. Mixing of the 2s and 2px orbitals is greater in B2, C2, and N2 than in O2, F2, and Ne2,because in B, C, and N the energy levels of the atomic orbitals are more closely spaced than in O,F, and Ne. This mixing produces a change in energy ordering for the π2p and the σ2p molecular orbitals.

In a Lewis model, the two bonds in a double bond look identical. However, valence bond theory shows that they are not. Describe a double bond according to valence bond theory. Explain why rotation is restricted about a double bond, but not about a single bond.

The double bond in Lewis theory is simple two pairs of electrons that are shared between the same two atoms. However, in valence bond theory we see that the double bond is made up of two different kinds of bonds. The double bond in valence bond theory consists of one sigma bond and one pi bond. Valence bond theory shows us that rotation about a double bond is severely restricted. Because of the side-by-side overlap of the p orbitals, the pi bond must essentially break for rotation to occur. The single bond consists of overlap that results in a pi bond. Since the overlap is linear, rotation is not restricted.

What is the role of wave interference in determining whether a molecular orbital is bonding or antibonding?

The electrons in orbitals behave like waves. The bonding molecular orbital arises from the constructive interference between the atomic orbitals and is lower in energy than the atomic orbitals. The antibonding molecular orbital arises from the destructive interference between the atomic orbitals and is higher in energy than the atomic orbitals.

When applying molecular orbital theory to heteronuclear diatomic molecules, the atomic orbitals used may be of different energies. If two atomic orbitals of different energies make two molecular orbitals, how are the energies of the molecular orbitals related to the energies of the atomic orbitals? How is the shape of the resultant molecular orbitals related to the shapes of the atomic orbitals?

When two atomic orbitals are different, the weighting of each orbital in forming a molecular orbital may be different. When a molecular orbital is approximated as a linear combination of atomic orbitals of different energies, the lower energy atomic orbital makes a greater contribution to the bonding molecular orbital and the higher energy atomic orbital makes a greater contribution to the antibonding molecular orbital. The shape of the molecular orbital shows a greater electron density at the atom that has the lower atomic orbital energy.

Name the hybridization scheme that corresponds to each electron geometry: a. linear b. trigonal planar c. tetrahedral d. trigonal bipyramidal e. octahedral

a. A linear electron geometry corresponds to sp hybridization. b. A trigonal planar electron geometry corresponds to sp2 hybridization. c. A tetrahedral electron geometry corresponds to sp3 hybridization. d. A trigonal bipyramidal electron geometry corresponds to sp3d hybridization. e. An octahedral electron geometry corresponds to sp3d2 hybridization.