10.2.3 Pi Bonds

Question 1

Pi Bonds

Answer 1

• 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.

Question 2

note

Answer 2

• 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 (CH 2 O) 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 (CO 2) 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.

Question 3

Which statement best describes what happens to the carbon atom in formaldehyde as a result of the hybridization process?

Answer 3

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.

Question 4

The formaldehyde molecule has a π bond between the carbon and oxygen atom. Which of the following statements about this bond is NOT true?

Answer 4

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.

Question 5

Which statement correctly correlates σ or π bonding atoms and their respective bonding orbitals in formaldehyde?

Answer 5

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.

Question 6

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?

Answer 6

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.

Question 7

Which statement correctly states what is “left over” after the σ bonds have formed in formaldehyde?

Answer 7

There is one 2pz orbital on the oxygen atom and one 2pz orbital on the carbon atom. Each orbital has an unpaired electron.

Question 8

Which of the following is equal to the total number of hybrid orbitals around a hybridized atom?

Answer 8

the steric number, SN

Question 9

What happens to the “leftover” (after σ bonding) orbitals in formaldehyde?

Answer 9

They form a π bond between the carbon atom and the oxygen atom.

Question 10

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?

Answer 10

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.

Question 11

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?

Answer 11

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.

Question 12

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?

Answer 12

An atom always has to have at least one available orbital with one unpaired electron in order to form a bond.