Beyond Homonuclear Diatomics (10.2.6) Flashcards
• Molecules containing atoms of similar electronegativities will have orbital energies that vary only slightly from homonuclear diatomic molecules.
• Molecules containing atoms of similar electronegativities will have orbital energies that vary only slightly from homonuclear diatomic molecules.
• Heteronuclear diatomics of very different electronegativities require the construction of a molecular orbital energy ladder appropriate to the placement of the valence electrons.
• Heteronuclear diatomics of very different electronegativities require the construction of a molecular orbital energy ladder appropriate to the placement of the valence electrons.
• Electrons in delocalized bonds can move throughout the atoms of a given molecule, providing a low energy configuration with increased stability.
• Electrons in delocalized bonds can move throughout the atoms of a given molecule, providing a low energy configuration with increased stability.
Carbon monoxide is an example of a heteronuclear
diatomic molecule. There are a total of 10 valence
electrons in carbon monoxide.
Combining the electrons of the 2s and 2p atomic
orbitals of carbon and oxygen to form molecular
orbitals predicts a bond order of 3, indicating that
there is a triple bond between carbon and oxygen.
The triple bond is formed from a sigma bond
between two of the p orbitals and two pi bonds
between the remaining p orbitals.
Since oxygen is more electronegative than carbon,
the oxygen orbitals are slightly lower in energy than
the carbon orbitals.
Hydrogen fluoride (HF) is a heteronuclear diatomic
molecule with a very large difference in
electronegativity between hydrogen and fluorine.
This large difference in electronegativity causes the
atomic orbitals of fluorine to be much lower in
energy than the atomic orbitals of hydrogen.
The best bonding interaction that occurs between
hydrogen and fluorine takes place between the 1s
orbital of hydrogen and the 2p orbital of fluorine that
is oriented towards hydrogen. These two atomic
orbitals form a sigma bonding orbital (σ) as well as
a sigma antibonding orbital (σ*).
The 2s orbital of fluorine and the other two 2p
orbitals of fluorine remain the same and may still
hold electrons.
All eight valence electrons from HF are then used to
fill the molecular orbital energy ladder. Most of the
electrons (two 2s and four 2p electrons) are
associated with F, while the two electrons in the
sigma bond are associated with both H and F.
Carbon monoxide is an example of a heteronuclear
diatomic molecule. There are a total of 10 valence
electrons in carbon monoxide.
Combining the electrons of the 2s and 2p atomic
orbitals of carbon and oxygen to form molecular
orbitals predicts a bond order of 3, indicating that
there is a triple bond between carbon and oxygen.
The triple bond is formed from a sigma bond
between two of the p orbitals and two pi bonds
between the remaining p orbitals.
Since oxygen is more electronegative than carbon,
the oxygen orbitals are slightly lower in energy than
the carbon orbitals.
Hydrogen fluoride (HF) is a heteronuclear diatomic
molecule with a very large difference in
electronegativity between hydrogen and fluorine.
This large difference in electronegativity causes the
atomic orbitals of fluorine to be much lower in
energy than the atomic orbitals of hydrogen.
The best bonding interaction that occurs between
hydrogen and fluorine takes place between the 1s
orbital of hydrogen and the 2p orbital of fluorine that
is oriented towards hydrogen. These two atomic
orbitals form a sigma bonding orbital (σ) as well as
a sigma antibonding orbital (σ*).
The 2s orbital of fluorine and the other two 2p
orbitals of fluorine remain the same and may still
hold electrons.
All eight valence electrons from HF are then used to
fill the molecular orbital energy ladder. Most of the
electrons (two 2s and four 2p electrons) are
associated with F, while the two electrons in the
sigma bond are associated with both H and F.
The orbital diagram for butadiene shows the four
possible orbital arrangements. The lowest energy
arrangement (bottom of diagram) has no
antibonding interactions. The next orbital has one
antibonding interaction. The third orbital
arrangement has two, and the highest energy
arrangement has three antibonding interactions.
Electrons are placed in the orbitals from lowest
energy to highest. Since there are four electrons in
the system (one per p orbital) they fill only the
bonding orbitals.
The orbital diagram predicts that the bond between
the middle two carbons has some double bond
character. It also predicts that the molecule is
planar, allowing all four p orbitals to line up.
A molecule with overlapping pi bonds could have
electrons delocalized across the entire length of the
molecule.
The orbital diagram for butadiene shows the four
possible orbital arrangements. The lowest energy
arrangement (bottom of diagram) has no
antibonding interactions. The next orbital has one
antibonding interaction. The third orbital
arrangement has two, and the highest energy
arrangement has three antibonding interactions.
Electrons are placed in the orbitals from lowest
energy to highest. Since there are four electrons in
the system (one per p orbital) they fill only the
bonding orbitals.
The orbital diagram predicts that the bond between
the middle two carbons has some double bond
character. It also predicts that the molecule is
planar, allowing all four p orbitals to line up.
A molecule with overlapping pi bonds could have
electrons delocalized across the entire length of the
molecule.
Characteristics of bonding that only molecular orbital theory can explain:
Metals - explains why metals are shiny
solids with band stricture
organic dyes - gives dyes vivid colors
organic solvents - makes them aromatic
To account for the high electronegativity difference between hydrogen and fluorine, we reassign the orbital sites in HF in order to create an energy ladder that gives us a logical sequence for assigning electrons. Which statement about the reassigned orbital sites in HF is true?
There are two molecular orbital sites and three atomic orbital sites. (B)
The two molecular orbital sites are the σ and σ* sites. The three atomic orbital sites are the two 2p sites and the one 2s site.
Which of the following is not characteristic of HF?
HF involves just atomic orbitals because the hydrogen atom just has one valence electron. (D)
This statement is incorrect. A σ bond is formed in this molecule.
We learned that four new molecular orbitals are created from the p orbitals for each carbon atom in the butadiene molecule (the axes perpendicular to the page). Which statement about these molecular orbitals is NOT true?
The four molecular p orbitals of the four carbon atoms can be lined up parallel in order to create four new molecular orbitals. (A)
This statement would be correct if you replaced four molecular orbitals with four atomic orbitals.
In our first approach to assigning electrons to orbital sites in the CO molecule, we used the same technique that we used before with N2 and O2. Which of the following steps is not part of this first approach for assigning electrons in CO?
Assign the ten valence electrons to the bonding orbital, 2s and 2p sites. (C)
This is almost correct. The term bonding orbital is wrong. It should not restrict the electron assignments to just bonding orbital sites. Anti-bonding orbitals are used as well.
What is the most important extra step that has to be included in the first approach technique for assigning electrons in a heteronuclear diatomic molecule?
Account for the electronegativity (and orbital energy) differences between the two atoms. (D)
The differences in electronegativity values can cause energy differences between atoms.
Which statement about molecular orbital theory is not true?
Molecular orbital theory leads to more accurate bond order values. (D)
What does occur is that bond order values are confirmed and better explained by molecular orbital theory.
Which statement about molecular bond theory is not true?
Molecular bond theory rejects valence bond theory and explains the bonding exclusively in terms of molecular (not atomic) bonding. (A)
This statement is not true. Molecular bond theory actually allows for atomic bonding explanations to be used when they’re appropriate. However, it also gives us a better tool for explaining unique bonding situations in molecules.
Which statement gives the best definition for isoelectronic molecules?
Isoelectronic molecules are molecules that have the same number of valence electrons and very similar orbitals that, as a result, give the molecules the same shape. (C)
CO2 and N2 are isoelectronic because they have the same number of valence electrons (ten), very similar orbitals, and identical shape.
Which statement best explains why the bonding between the hydrogen and fluorine atoms occurs where it does?
The fluorine atom has such lower bonding energy levels that its 2px orbital is the one that is best poised for bonding with the 1s orbital of hydrogen. (D)
The important distinction is to note that fluorine has much lower bond energy levels than hydrogen. This causes the whole bond energy ladder to “tilt” significantly.
