10.2.1 Valence Bond Theory

Question 1

Valence Bond Theory

Answer 1

• When two atoms are in proximity, they will arrange themselves at the lowest possible energy level.
• Sigma bonds are formed by the end-on overlap of orbitals along the internuclear axis.
• Pi bonds are formed by side-on overlap of orbitals on either side of the internuclear axis.

Question 2

note

Answer 2

• As the distance between two hydrogen atoms decreases, the attraction between them increases. The reason is that the nucleus of one atom can attract the electrons of the other atom. Beyond a critical point, however, the nuclei will repel each other. The diagram on the left depicts the optimal point of attraction between the atoms, a distance of 74 picometers. At this distance, the energy of the electrons is lower than at any other distance. Atoms always seek the lowest possible energy level.
• Valence bond theory states that a chemical bond results from the overlap of two orbitals, each of which is half filled. With respect to dihydrogen, the electron from each orbital occupies the other hydrogen’s orbital because two electrons represent the most stable configuration, forming what is known as a sigma bond. In contrast, helium doesn’t form a bond because there is not a place for another electron. The 1s orbital in helium contains two electrons and consequently cannot accept the electrons of another atom.
• Sigma bonds form when two s orbitals overlap, as in
  dihydrogen, or when an orbital that has directional character, such as a p orbital, overlaps another orbital along the direction of the orbital axis. For example, the p z orbital of fluorine overlaps a 1s orbital along its axis, resulting in a sigma bond.
• Pi bonds form by the sideways overlap of two parallel p orbitals. Pi bonds are not as strong as sigma bonds.
• Nitrogen has an electron configuration of 1s 2 2s 2 2p 3 . Because there are three unpaired electrons in nitrogen, three bonds can be made. There is one sigma bond and two pi bonds. There is side-on overlap along both the x and y axes (the two pi bonds) and end-on overlap along the z-axis (the sigma bond). This bond is a triple bond, as predicted by Lewis dot structure.

Question 3

Suppose that a theoretical gas atom has an electron configuration of 1s22s22p6. Which statement best describes this atom’s electron arrangement and theoretical reactivity with an atom that has an unpaired electron?

Answer 3

This atom has five orbitals, 1s, 2s, and three 2p orbitals. They each contain paired electrons. This atom will not display highly reactive tendencies, because its orbitals are already filled

Question 4

Which statement about the chemical bond between two hydrogen atoms is not true?

Answer 4

The amount of energy needed to break the bond is twice the potential energy of the ideally bonded atoms.

Question 5

Oxygen has an electron configuration of 1s2 2s2 2p4. Which statement about oxygen is not correct?

Answer 5

An oxygen atom would be less likely to form a bond than an atom that has an electron configuration of 1s2 2s2 2p6.

Question 6

The bonding behavior of two hydrogen atoms can be observed in the graph.

Which of the following statements is completely correct?

Answer 6

Electron sharing actually can reduce the potential energy of the system, which in this case is H2. The potential energy is minimized at an ideal internuclear separation distance of 74 pm. At any other internuclear distance you do not have minimal potential energy for the system.

Question 7

Which statement about valence bond theory is not true?

Answer 7

An atom with a filled valence shell is unstable and therefore very willing to form a covalent bond with another atom

Question 8

Which statement best describes the bonding in an O2 molecule?

Answer 8

O2 has one σ bond and one π bond.

Question 9

Which statement about the bonds in N2 is not correct? Remember that the electron configuration for nitrogen in the ground state is 1s22s22p3.

Answer 9

There are two σ bonds and one π bond. The σ bonds are arranged along the x- and y-axes, and the π bond is arranged along the z-axis.

Question 10

Look at the graph that compares the internuclear separation between two hydrogen atoms with their potential energy. What is indicated by Point 1 on the graph?

Answer 10

This is the point at which the two hydrogen atoms have the lowest potential energy and a defined internuclear axis length.

Question 11

The electron configuration for fluorine, F, is 1s2 2s2 2p5 . Which statement about the electron configuration for fluorine is not correct?

Answer 11

All the orbitals in the fluorine atom have complete electron pairing.

Question 12

What is the difference between a σ bond and a π bond?

Answer 12

A σ bond’s overlap occurs along the internuclear axis.