Pi Bonds (10.2.3) Flashcards
• A pi molecular orbital is a molecular orbital created from the side-on overlap of unhybridized atomic p orbitals
perpendicular to the plane of the molecule.
• A pi molecular orbital is a molecular orbital created from the side-on overlap of unhybridized atomic p orbitals
perpendicular to the plane of the molecule.
• Pi molecular orbitals are used to form pi bonds, which are involved in double bonds and triple bonds.
• Pi molecular orbitals are used to form pi bonds, which are involved in double bonds and triple bonds.
A pi molecular orbital is a molecular orbital created from the
side-on overlap of unhybridized atomic p orbitals
perpendicular to the plane of the molecule.
For example, formaldehyde (CH2O) contains a double bond
between carbon and oxygen. Two of the electrons in this
bond are in a sigma molecular orbital, and the other two
are in a pi molecular orbital. This orbital is formed from the
unhybridized p orbital of carbon and one of the unhybridized
p orbitals of oxygen.
A pi molecular orbital is a molecular orbital created from the
side-on overlap of unhybridized atomic p orbitals
perpendicular to the plane of the molecule.
For example, formaldehyde (CH2O) contains a double bond
between carbon and oxygen. Two of the electrons in this
bond are in a sigma molecular orbital, and the other two
are in a pi molecular orbital. This orbital is formed from the
unhybridized p orbital of carbon and one of the unhybridized
p orbitals of oxygen.
Pi molecular orbitals are used to form pi bonds, which are
involved in double bonds and triple bonds.
For example, the carbon atom in carbon dioxide (CO2
) has
two perpendicular 2p orbitals. One of these orbitals forms a
pi bond with a 2p orbital of the oxygen atom on the left, and
the other forms a pi bond with a 2p orbital of the oxygen atom
on the right. The remaining bond of each double bond in
carbon dioxide is a sigma bond.
Pi molecular orbitals are used to form pi bonds, which are
involved in double bonds and triple bonds.
For example, the carbon atom in carbon dioxide (CO2
) has
two perpendicular 2p orbitals. One of these orbitals forms a
pi bond with a 2p orbital of the oxygen atom on the left, and
the other forms a pi bond with a 2p orbital of the oxygen atom
on the right. The remaining bond of each double bond in
carbon dioxide is a sigma bond.
What happens to the “leftover” (after σ bonding) orbitals in formaldehyde?
They form a π bond between the carbon atom and the oxygen atom. (C)
This is the correct answer. Remember that a σ bond between the carbon atom and the oxygen atom already exists. So the two atoms will form a π bond with their remaining unpaired electrons.
Look at the electron configurations for an oxygen atom and a carbon atom.
Which statement best describes the bond tendencies for both of the atoms?
It appears that the oxygen atom can form two bonds because it has two unpaired electrons in the 2p orbitals. It appears that the carbon atom will want to alter its electron distribution in order to have more bonding opportunities with other atoms. (B)
This is a very comprehensive and informative statement that deals with both the oxygen and carbon atoms.
Which statement correctly states what is “left over” after the σ bonds have formed in formaldehyde?
There is one 2pz orbital on the oxygen atom and one 2pz orbital on the carbon atom. Each orbital has an unpaired electron. (A)
The oxygen (O) atom has two lone electrons in its 2py and 2pz orbitals. The 2py orbital is used to form the σ bond with oxygen, leaving the 2pz orbital. In the carbon (C) atom the original four electrons are given new lone electron assignments. Three of them are assigned to the three hybrid sp2 orbitals, and the fourth one is assigned to the standard 2pz orbital in the atom.
Which statement best describes what happens to the carbon atom in formaldehyde as a result of the hybridization process?
Carbon’s 2s and 2p orbitals are hybridized, resulting in three hybrid sp2 orbitals and one (non-hybridized) pz orbital. Each new orbital has one lone electron assigned to it. (D)
Three of the original four orbitals (one 2s orbital and two of the 2p orbitals) are hybridized, while only one of the original four orbitals (the pz orbital) remains in its original orbital state.
We have learned that valence bond theory helps us to predict σ and π bonding in molecules. All of the following statements about molecular bonding are true, but which statement best summarizes the most basic premise for bonding in a molecule?
An atom always has to have at least one available orbital with one unpaired electron in order to form a bond. (B)
Although all four statements are true, this one best addresses the very basic, necessary circumstance for bonding: the need for one available orbital and one unpaired electron in order to form a bond.
The formaldehyde molecule has a π bond between the carbon and oxygen atom. Which of the following statements about this bond is NOT true?
This bond is the result of the interaction of an unpaired carbon electron in the 2s orbital and an unpaired oxygen electron in the 2p orbital. (C)
The bond is the result of unpaired electrons from 2p sites in both atoms interacting to form a bond.
Consider the more complicated bonding that occurs in carbon dioxide, CO2.
Which of the following best predicts the bonding orbitals that will be used in the formation of a carbon dioxide molecule?
Since the SN for carbon (in carbon dioxide) = 2, the carbon will be hybridized in such a way as to create two sp hybrid orbitals and two regular 2p orbitals. There will be one unpaired electron in each orbital orbital. (D)
This is an accurate statement for what is occurring. At each bonding site, the hybridized carbon will be poised to form two bonds. This is because the hybridized carbon atom will have a hybrid sp orbital and a regular 2p orbital (each with an unpaired electron) available at each site.
The carbon dioxide molecule, CO2, has just two bonding sites. There is a double bond that occurs at each site. Which statement correctly predicts and describes the bonding arrangement for CO2?
Since the SN for the carbon atom in CO2 is 2, there are only two bonding sites between the central carbon atom and the surrounding two oxygen atoms. As a result, a double bond, consisting of a σ bond and a π bond, will form at each site. (B)
This is correct and a thorough explanation of what happens in the carbon dioxide molecule. If you look at the electron configuration for oxygen, you can see that there are two unpaired electrons available for bonding for each oxygen atom. Since there are two oxygen atoms, there are a total of four available unpaired electrons for bonding. If you look at the hybridized carbon atom for this molecule, you can see that there are two unpaired electrons in the two sp hybrid orbitals and two unpaired electrons in the two p orbitals (for a total of four unpaired electrons). So there are four unpaired electrons available from the oxygen atoms and four unpaired electrons available from the hybridized carbon atom. This means four total bonds will result.
Which of the following is equal to the total number of hybrid orbitals around a hybridized atom?
the steric number, SN. (B)
SN does tell you the number of hybrid orbitals involved in a hybrid. It is a very helpful tool for predicting molecular geometry of molecules with hybridized central atoms.
Which statement correctly correlates σ or π bonding atoms and their respective bonding orbitals in formaldehyde?
The oxygen atom (with its 2p orbital) forms a σ bond with one of the three sp2 hybrid orbitals of the carbon atom; the two hydrogen atoms (with their 1s orbitals) each form a σ bond with one of the three sp2 hybrid orbitals of the carbon atom. The oxygen atom also forms a π bond with carbon which involves the 2pz orbitals of oxygen and carbon. (D)
This is the correct answer. In the oxygen atom there are two 2p sites available for bonding. In the hybridized carbon atom there is an unpaired electron assigned to each of three sp2 hybrid orbitals and one pz orbital.
