Chapter 6: Chemical Bonding II

Question 1

How does the definition of a chemical bond differ between the Lewis model and valence bond theory?

Answer 1

In the Lewis model, a chemical bond is a shared electron pair. In valence bond theory, a chemical bond is the overlap between two half-filled atomic orbitals. While the Lewis model represents valence electrons as dots, the valence bond theory represents valence electrons as residing in quantum-mechanical atomic orbitals.

Question 2

Why does a chemical bond form?

Answer 2

A chemical bond results from the overlap of two half-filled orbitals and spin-pairing of the two valence electrons.

Question 3

When interacting atomic orbitals contain a total of two electrons that can spin-pair (orient with opposing spins), what happens to the interaction energy?

Answer 3

At this point, the interaction energy is usually negative or stabilizing.

Question 4

Define hybridization.

Answer 4

Hybridization is a mathematical procedure in which the standard atomic orbitals are combined to form new atomic orbitals called hybrid orbitals that correspond more closely to the actual distribution of electrons in chemically bonded atoms. Hybrid orbitals are still localized on individual atoms, but they have different shapes and energies from those of standard orbitals.

Question 5

What is the relationship between the overlap of orbitals and the energy and strength of the bond?

Answer 5

The greater the overlap, the lower the energy and the stronger the bond.

Question 6

Why does valence bond theory propose that electrons in some molecules occupy hybrid orbitals instead of the standard atomic orbitals?

Answer 6

Hybrid orbitals allow greater overlap because the electron density in a hybrid orbital is concentrated along a single directional lobe. This concentration of electron density in a single direction allows for greater overlap between orbitals. In other words, hybrid orbitals minimize the energy of the molecule by maximizing the orbital overlap in a bond.

Question 7

What is the general relationship between the bonds an atom forms and its tendency to hybridize its orbitals?

Answer 7

Hybridization costs energy. So, hybridization occurs only to the degree that the energy payback through bond formation is large. Therefore, the more bonds that an atom forms, the greater the tendency of its orbitals to hybridize.

Question 8

Are central or terminal atoms more likely to hybridize their bonds?

Answer 8

Central or interior atoms are more likely to hybridize their bonds because they form more bonds than most terminal atoms.

Question 9

What are the three rules for hybridization?

Answer 9

1. The number of standard atomic orbitals added together always equals the number of hybrid orbitals formed. The total number of orbitals is conserved.
2. The particular combinations of standard atomic orbitals added together determine the shapes and energies of the hybrid orbitals formed.
3. The particular type of hybridization that occurs is the one that yields the lowest overall energy for the molecule.

Question 10

How does the presence of lone pairs affect hybridization?

Answer 10

The presence of a lone pair lowers the tendency of orbitals to hybridize since the tendency to hybridize increases with the number of bonds formed.

Question 11

What is the difference between a pi and sigma bond?

Answer 11

Orbitals overlap side-by-side to form a pi bond. Orbitals overlap end-to-end to form a sigma bond.

Question 12

How many sigma bonds can any two atoms have?

Answer 12

There can only be one sigma bond. Any additional bonds must be pi bonds.

Question 13

How do the strengths of pi and sigma bonds compare?

Answer 13

In general, pi bonds are weaker than sigma bonds because the side-to-side orbital overlap tends to be less efficient than the end-to-end orbital overlap. Consequently, the pi bond in a double bond is generally easier to break than the sigma bond.

Question 14

Distinguish between cis and trans forms of a molecule.

Answer 14

In reference to the spatial arrangement of atoms, cis means same side and trans means opposite ends. These two forms (called isomers) of a molecule have different structures and therefore different properties.

Question 15

What does the hybridization of one s and one p orbital result in?

Answer 15

Two sp hybrid orbitals and two leftover unhybridized p orbitals. This hybridization has a linear geometry.

Question 16

What does the hybridization of one s and two p orbitals result in?

Answer 16

Three sp^2 hybrids and one leftover unhybridized p orbital. This hybridization has trigonal planar geometry.

Question 17

What does the hybridization of one s orbital, three p orbitals, and one d orbital result in?

Answer 17

Five sp^3d hybrid orbitals with trigonal bipyramidal geometry.

Question 18

What does the hybridization of one s orbital, three p orbitals, and two d orbitals result in?

Answer 18

Six sp^3d^2 hybrid orbitals with octahedral geometry.

Question 19

If the central atom of a molecule is bound to two electron groups, what is its electron geometry and hybridization scheme?

Answer 19

Linear sp.

Question 20

If the central atom of a molecule is bound to three electron groups, what is its electron geometry and hybridization scheme?

Answer 20

Trigonal planar sp^2.

Question 21

If the central atom of a molecule is bound to four electron groups, what is its electron geometry and hybridization scheme?

Answer 21

Tetrahedral sp^3.

Question 22

If the central atom of a molecule is bound to five electron groups, what is its electron geometry and hybridization scheme?

Answer 22

Trigonal bipyramidal sp^3d.

Question 23

If the central atom of a molecule is bound to six electron groups, what is its electron geometry and hybridization scheme?

Answer 23

Octahedral sp^3d^2.

Question 24

How does the energy of electrons in bonding molecular orbitals compare to the energy of electrons in atomic orbitals?

Answer 24

When electrons occupy bonding molecular orbitals, the energy of the electrons is lower than it would be if they were occupying atomic orbitals.

Question 25

Define LCAO.

Answer 25

An LCAO is a linear combination of atomic orbitals. An LCAO molecular orbital is a weighted linear sum of the valence atomic orbitals of the atoms in the molecule.

Question 26

How are the hybrid orbitals of valence bond theory different from LCAOs?

Answer 26

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 and 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.

Question 27

How does the energy of electrons in antibonding molecular orbitals compare to the energy of electrons in atomic orbitals?

Answer 27

Electrons in antibonding orbitals have higher energies than they had in their respective atomic orbitals and therefore tend to raise the energy of the system (relative to the unbonded atoms).

Question 28

How does wave interference determine the bonding or antibonding nature of a molecular orbital?

Answer 28

The bonding molecular orbital arises out of constructive interference between the atomic orbitals because both orbitals have the same phase. The antibonding orbital arises out of destructive interference between the atomic orbitals because subtracting one from the other means the two interacting orbitals have opposite phases.

Question 29

Compare the electron density of the internuclear region of bonding and antibonding orbitals. How does this affect energy?

Answer 29

The bonding orbital has an increased electron density in the internuclear region while the antibonding orbital has a node there. The greater electron density for a bonding orbital lowers its energy while the diminished electron density of the antibonding orbital increases its energy.

Question 30

How is the bond order of a diatomic molecule calculated?

Answer 30

bond order = (number of electrons in bonding MOs) - (number of electrons in antibonding MOs) all divided by 2.

Question 31

What does the bond order tell you about the strength of a bond and the likeliness of its formation?

Answer 31

A positive bond order means that there are more electrons in bonding molecular orbitals than in antibonding molecular orbitals. The electrons therefore have lower energy than they had in the orbitals of the isolated atoms, and a chemical bond forms. In general, 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.

Question 32

What three things does Lewis theory fail to explain that valence bond theory and molecular orbital theory can?

Answer 32

1. It cannot explain the rigidity of double and triple bonds.
2. Cannot accurately predict magnetic properties.
3. Certain VSEPR geometries not possible using s, p, or d orbitals.

Question 33

In valence bond theory, what condition makes the energy associated with two atoms coming together negative?

Answer 33

The interacting atomic orbitals contain two electrons capable of spin-pairing. This is usually seen as two, half-filled orbitals coming together.

Question 34

What is a coordinate covalent bond?

Answer 34

A fully occupied atomic orbital on one atom that overlaps with an empty atomic orbital on the other.

Question 35

How does valence bond theory fail?

Answer 35

VBT predicts that carbon should bond just like sulfur does. It has two unpaired electrons, thus it should make CX2 compounds in a bent structure with 90 degree bond angles. However, we know that C actually forms CX4 compounds in a tetrahedral structure with 109.5 degree bond angles. With VBT, the trigonal planar, tetrahedral, and trigonal bipyramidal structures shouldn’t even be possible (yet they exist).

Question 36

Define hybridization.

Answer 36

Hybridization is a mathematical procedure whereby atomic orbitals are combined to create hybrid orbitals, which are superpositions (overlaps) of atomic orbitals. They minimize energy by maximizing orbital overlap and adjusting orbital orientation.

Question 37

What electronic geometric orientation do sp hybridized orbitals form?

Answer 37

Linear.

Question 38

What electronic geometric orientation do sp2 hybridized orbitals form?

Answer 38

Trigonal planar

Question 39

What electronic geometric orientation do sp3 hybridized orbitals form?

Answer 39

Tetrahedral.

Question 40

What electronic geometric orientation do sp3d hybridized orbitals form?

Answer 40

Trigonal-bipyramidal.

Question 41

What electronic geometric orientation do sp3d2 hybridized orbitals form?

Answer 41

Octahedral.

Question 42

Give three important characteristics of sigma bonds.

Answer 42

1. Result from the head-on overlap of any two orbitals.
2. The resulting bond is located on the bonding axis.
3. All single bonds, as well as the first bond in double and triple bonds, are sigma bonds.

Question 43

Give four important characteristics of pi bonds.

Answer 43

1. Result from the side-on overlap of two orbitals.
2. S-orbitals cannot from a pi bond.
3. Electron density located above and below the bond axis. The bond axis is a node.
4. The 2nd bond of a double bond and the 2nd and 3rd bonds in a triple bond are pi bonds.

Question 44

For any stable molecule, how many bonding molecular orbitals are there for every anti-bonding molecular orbital?

Answer 44

For any stable molecule, the number of bonding MOs is equal to the number of anti-bonding MOs. They may have differing numbers of electron in the orbitals, but the number of orbitals themselves are the same.

Question 45

Define geometric isomers.

Answer 45

Geometric isomers are molecules that have the same formula but different atomic arrangements around a bond. The two types are cis (same side) and trans (opposite side). Geometric isomers have different chemical and physical properties from one another.

Question 46

According to molecular orbital theory, what happens if two orbitals overlap in-phase?

Answer 46

They create a bonding orbital as a result of constructive interference. The orbital is lower in energy than the two atomic orbitals used to make it.

Question 47

According to molecular orbital theory, what happens if two orbitals overlap out-of-phase?

Answer 47

They create an anti-bonding orbital with a node as a result of destructive interference. The orbital is higher in energy than the two atomic orbitals used to make it.

Question 48

What does resonance look like in a molecule as described using molecular orbital theory?

Answer 48

In MO theory, molecular orbitals are constructed with atomic orbitals from all of the atoms, meaning an MO can span the entire molecule. This delocalizes the electrons.

Question 49

How does the strength of a bond change with its length?

Answer 49

Weaker bonds are longer bonds. For example, a bond order of 0.5 is weaker than a bond order of 1, meaning the 0.5 bond is also longer.

Question 50

What is the Z>7 rule?

Answer 50

This rule says that the relative energy of the pi (2p) MO and sigma (2p) MO switch as you move from N2 (Z=7) to O2 (Z=8). When drawing a MO diagram for B, C, or N, the bonding order is different than for O, F, or Ne.

Question 51

When drawing the orbital diagram for a heteronuclear diatomic molecule, which atom determines the trend (i.e. is it affected by the Z>7 rule?)?

Answer 51

In heteronuclear diatomic molecules, follow the trend of the more electronegative atom.

Question 52

Describe the special bonding of F with H according to MOT.

Answer 52

F is so electronegative compared to H that the only orbitals of similar energy are H 1s and F 2p.