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3.11 Molecular orbital theory  (Page 7/26)

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As we saw in valence bond theory, σ bonds are generally more stable than π bonds formed from degenerate atomic orbitals. Similarly, in molecular orbital theory, σ orbitals are usually more stable than π orbitals. However, this is not always the case. The MOs for the valence orbitals of the second period are shown in [link] . Looking at Ne 2 molecular orbitals, we see that the order is consistent with the generic diagram shown in the previous section. However, for atoms with three or fewer electrons in the p orbitals (Li through N) we observe a different pattern, in which the σ p orbital is higher in energy than the π p set. Obtain the molecular orbital diagram for a homonuclear diatomic ion by adding or subtracting electrons from the diagram for the neutral molecule.

A graph is shown in which the y-axis is labeled, “E,” and appears as a vertical, upward-facing arrow. Across the top, the graph reads, “L i subscript 2,” “B e subscript 2,” “B subscript 2,” “C subscript 2,” “N subscript 2,” “O subscript 2,” “F subscript 2,” and “Ne subscript 2.” Directly below each of these element terms is a single pink line, and all lines are connected to one another by a dashed line, to create an overall line that decreases in height as it moves from left to right across the graph. This line is labeled, “sigma subscript 2 p x superscript asterisk”. Directly below each of these lines is a set of two pink lines, and all lines are connected to one another by a dashed line, to create an overall line that decreases in height as it moves from left to right across the graph. It is consistently lower than the first line. This line is labeled, “pi subscript 2 p y superscript asterisk,” and, “pi subscript 2 p z superscript asterisk.” Directly below each of these double lines is a single pink line, and all lines are connected to one another by a dashed line, to create an overall line that decreases in height as it moves from left to right across the graph. It has a distinctive drop at the label, “O subscript 2.” This line is labeled, “sigma subscript 2 p x.” Directly below each of these lines is a set of two pink lines, and all lines are connected to one another by a dashed line to create an overall line that decreases very slightly in height as it moves from left to right across the graph. It is consistently lower than the third line until it reaches the point labeled, “O subscript 2.” This line is labeled, “pi subscript 2 p y,” and, “pi subscript 2 p z.” Directly below each of these lines is a single blue line, and all lines are connected to one another by a dashed line to create an overall line that decreases in height as it moves from left to right across the graph. This line is labeled, “sigma subscript 2 s superscript asterisk.” Finally, directly below each of these lines is a single blue line, and all lines are connected to one another by a dashed line to create an overall line that decreases in height as it moves from left to right across the graph. This line is labeled. “sigma subscript 2 s.”
This shows the MO diagrams for each homonuclear diatomic molecule in the second period. The orbital energies decrease across the period as the effective nuclear charge increases and atomic radius decreases. Between N 2 and O 2 , the order of the orbitals changes.

You can practice labeling and filling molecular orbitals with this interactive tutorial from the University of Sydney.

This switch in orbital ordering occurs because of a phenomenon called s-p mixing    . s-p mixing does not create new orbitals; it merely influences the energies of the existing molecular orbitals. The σ s wavefunction mathematically combines with the σ p wavefunction, with the result that the σ s orbital becomes more stable, and the σ p orbital becomes less stable ( [link] ). Similarly, the antibonding orbitals also undergo s-p mixing, with the σ s* becoming more stable and the σ p* becoming less stable.

A diagram is shown. At the bottom left of the diagram is a horizontal line that is connected to the right and left by upward-facing, dotted lines to two more horizontal lines. Those two lines are connected by upward-facing dotted lines to another line in the center of the diagram but farther up from the first. Each of the bottom two central lines has a vertical downward-facing arrow. Above this structure is a horizontal line that is connected to the right and left by upward-facing, dotted lines to two sets of three horizontal lines and those two lines are connected by upward-facing dotted lines to another line in the center of the diagram, but further up from the first. In between the horizontal lines of this structure are two pairs of horizontal lines that are above the first line but below the second and connected by dotted lines to the side horizontal lines. The bottom and top central lines each have an upward-facing vertical arrow. These two structures are redrawn on the right side of the diagram, but this time, the central lines of the bottom structure are moved downward in relation to the side lines. The upper portion of the structure has its central lines shifted upward in relation to the side lines. This structure also shows the bottom line appearing above the set of two lines.
Without mixing, the MO pattern occurs as expected, with the σ p orbital lower in energy than the σ p orbitals. When s-p mixing occurs, the orbitals shift as shown, with the σ p orbital higher in energy than the π p orbitals.

s-p mixing occurs when the s and p orbitals have similar energies. When a single p orbital contains a pair of electrons, the act of pairing the electrons raises the energy of the orbital. Thus the 2 p orbitals for O, F, and Ne are higher in energy than the 2 p orbitals for Li, Be, B, C, and N. Because of this, O 2 , F 2 , and N 2 only have negligible s-p mixing (not sufficient to change the energy ordering), and their MO diagrams follow the normal pattern, as shown in [link] . All of the other period 2 diatomic molecules do have s-p mixing, which leads to the pattern where the σ p orbital is raised above the π p set.

Using the MO diagrams shown in [link] , we can add in the electrons and determine the molecular electron configuration and bond order for each of the diatomic molecules. As shown in [link] , Be 2 and Ne 2 molecules would have a bond order of 0, and these molecules do not exist.

Electron Configuration and Bond Order for Molecular Orbitals in Homonuclear Diatomic Molecules of Period Two Elements
Molecule Electron Configuration Bond Order
Li 2 ( σ 2 s ) 2 1
Be 2 (unstable) ( σ 2 s ) 2 ( σ 2 s * ) 2 0
B 2 ( σ 2 s ) 2 ( σ 2 s * ) 2 ( π 2 p y , π 2 p z ) 2 1
C 2 ( σ 2 s ) 2 ( σ 2 s * ) 2 ( π 2 p y , π 2 p z ) 4 2
N 2 ( σ 2 s ) 2 ( σ 2 s * ) 2 ( π 2 p y , π 2 p z ) 4 ( σ 2 p x ) 2 3
O 2 ( σ 2 s ) 2 ( σ 2 s * ) 2 ( σ 2 p x ) 2 ( π 2 p y , π 2 p z ) 4 ( π 2 p y * , π 2 p z * ) 2 2
F 2 ( σ 2 s ) 2 ( σ 2 s * ) 2 ( σ 2 p x ) 2 ( π 2 p y , π 2 p z ) 4 ( π 2 p y * , π 2 p z * ) 4 1
Ne 2 (unstable) ( σ 2 s ) 2 ( σ 2 s * ) 2 ( σ 2 p x ) 2 ( π 2 p y , π 2 p z ) 4 ( π 2 p y * , π 2 p z * ) 4 ( σ 2 p x * ) 2 0
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Source:  OpenStax, Ut austin - principles of chemistry. OpenStax CNX. Mar 31, 2016 Download for free at http://legacy.cnx.org/content/col11830/1.13
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