12. Molecular Shapes & Valence Bond Theory

MO Theory: Bond Order

1.4 Molecular Orbital Theory

Calcium carbide, CaC2, reacts with water to produce acetylene,
C2H2, and is sometimes used as a convenient source of
that substance. Use the MO energy diagram in Figure 8.22a
to describe the bonding in the carbide anion, C22-. What is
its bond order?

Using the molecular orbital energy ordering for second-row 
homonuclear diatomic molecules in which the p2p orbitals lie at 
lower energy than the s2p, draw MO energy diagrams and
predict the bond order in a molecule or ion with each number 
of total valence electrons. Will the molecule or ion be diamagnetic 
or paramagnetic? 
a. 4

Determine the electron configurations for CN+, CN, and 
CN-. (a) Which species has the strongest C¬N bond?

According to MO theory, which molecule or ion has the highest bond energy? 
O2, O2- , O22 -

According to MO theory, which molecule or ion has the shortest bond length? 
O2, O2- , O22 -

Apply molecular orbital theory to predict which species has the strongest bond.&nbsp;a) O2&nbsp;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; b) O2–&nbsp;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; c) O2+&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; d) All the bonds are equivalent&nbsp;

Use the MO energy diagram in Figure 8.22b to describe the
bonding in O2+, O2, and O2-. Which of the three is likely to
be stable? What is the bond order of each? Which contain
unpaired electrons?

One of the molecular orbitals of the H2- ion is sketched below:

(d) Compared to the H¬H bond in H2, the H¬H bond in H2- is expected to be which of the following:
(i) Shorter and stronger, (ii) longer and stronger, (iii) shorter
and weaker, (iv) longer and weaker, or (v) the same length
and strength?

Draw a molecular orbital energy diagram for ClF. (Assume that
the sp orbitals are lower in energy than the p orbitals.) What is
the bond order in ClF?

Molecular Orbital Theory III:  Bond Order and Stability

Apply Molecular Orbital Theory to determine the bond order of HHe+&nbsp;ion.

Explain the following: (c) The O22 + ion has a stronger O—O bond than O2 itself.

Place the following molecules and ions in order from smallest
to largest bond order: N22+, He2+, Cl2 H2-, O22-.

Carbon monoxide is produced by incomplete combustion of 
fossil fuels. 
(c) What is the bond order of CO? Does this match the 
bond order predicted by the electron-dot structure?

Using the molecular orbital energy ordering for second-row homonuclear 
diatomic molecules in which the p2p orbitals lie at 
higher energy than the s2p, draw MO energy diagrams and predict 
the bond order in a molecule or ion with each number of 
total valence electrons. Will the molecule or ion be diamagnetic 
or paramagnetic? 
c. 13

Use molecular orbital theory to predict if each molecule or ion 
exists in a relatively stable form. 
a. H22 -

Using Figures 9.35 and 9.43 as guides, draw the molecular orbital
electron configuration for (d) Ne22 +. In each case indicate whether the addition of an electron to the ion would increase or decrease the bond order of the species.

MO Theory: Bond Order Example 2

If an electron is removed to give CN+, the bond length decreases.

The π*2p orbital is the highest energy orbital containing an electron in CN.

If an electron is added to give CN–, the bond order increases.

Using Molecular Orbital Theory, answer the following questions dealing with carbon mononitride, CN.&nbsp;

Consider the H2+ ion. (d) What is the bond order in H2+?

Molecular Orbital Theory - Bonding & Antibonding MO - Bond Order

Using the molecular orbital energy ordering for second-row homonuclear 
diatomic molecules in which the p2p orbitals lie at 
higher energy than the s2p, draw MO energy diagrams and predict 
the bond order in a molecule or ion with each number of 
total valence electrons. Will the molecule or ion be diamagnetic 
or paramagnetic? 
a. 10

Introduction to Molecular Orbital Theory

Draw an MO energy diagram for CO. (Use the energy ordering 
of O2.) Predict the bond order and make a sketch of the lowest 
energy bonding molecular orbital.

Draw an MO energy diagram and predict the bond order of Li2+ and Li2-. Do you expect these molecules to exist in the gas phase?

MO Theory: Bond Order Concept 1

Draw an MO energy diagram and predict the bond order of Be2+ and Be2- . Do you expect these molecules to exist in the gas phase?

Draw an energy diagram for HCl. Predict the bond order and 
make a sketch of the lowest energy bonding molecular orbital.

If we assume that the energy-level diagrams for homonuclear 
diatomic molecules shown in Figure 9.43 can be applied 
to heteronuclear diatomic molecules and ions, predict the 
bond order and magnetic behavior of (d) ClF.

The C2 molecule has a MO diagram similar to N2 
(Figure 8.22a). What is the bond order of C2 and is it paramagnetic 
or diamagnetic? (LO 8.12) 
(a) Bond order = 2, diamagnetic 
(b) Bond order = 2, paramagnetic 
(c) Bond order = 0, paramagnetic 
(d) Bond order = 3>2, diamagnetic

The Molecular Orbital Theory.

Consider the H2+ ion. (e) Suppose that the ion is excited 
by light so that an electron moves from a lower-energy to 
a higher-energy MO. Would you expect the excited-state 
H2+ ion to be stable or to fall apart?

Using the molecular orbital energy ordering for second-row 
homonuclear diatomic molecules in which the p2p orbitals lie at 
lower energy than the s2p, draw MO energy diagrams and 
predict the bond order in a molecule or ion with each number 
of total valence electrons. Will the molecule or ion be diamagnetic 
or paramagnetic? 
d. 9

Using the molecular orbital energy ordering for second-row homonuclear 
diatomic molecules in which the p2p orbitals lie at 
higher energy than the s2p, draw MO energy diagrams and predict 
the bond order in a molecule or ion with each number of 
total valence electrons. Will the molecule or ion be diamagnetic 
or paramagnetic? 
d. 14

The C2 molecule can be represented by an MO diagram similar 
to that in Figure 8.22a. 
(b) To increase the bond order of C2, should you add or 
remove an electron?

MO Theory: Bond Order Example 1

According to MO theory, which molecule or ion has the highest 
bond order? 
O2, O2- , O22 -

Using the molecular orbital energy ordering for second-row 
homonuclear diatomic molecules in which the p2p orbitals lie at 
lower energy than the s2p, draw MO energy diagrams and 
predict the bond order in a molecule or ion with each number 
of total valence electrons. Will the molecule or ion be diamagnetic 
or paramagnetic? 
c. 8

Draw Lewis structures and MO diagrams for CN+ , CN, and 
CN- . According to the Lewis model, which species is most stable?

The energy-level diagram in Figure 9.36 shows that the 
sideways overlap of a pair of p orbitals produces two molecular 
orbitals, one bonding and one antibonding. In ethylene there 
is a pair of electrons in the bonding π orbital between the two 
carbons. Absorption of a photon of the appropriate wavelength 
can result in promotion of one of the bonding electrons 
from the p2p to the p*2p molecular orbital. (c) Is the C¬C bond 
in ethylene stronger or weaker in the excited state than in the 
ground state? Why?

If we assume that the energy-level diagrams for homonuclear
diatomic molecules shown in Figure 9.43 can be applied
to heteronuclear diatomic molecules and ions, predict the
bond order and magnetic behavior of (b) NO+.

At high temperatures, sulfur vapor is predominantly in the 
form of S21g2 molecules. 
(d) When two electrons are added to S2, the disulfide ion 
S22- is formed. Is the bond length in S22- likely to be 
shorter or longer than the bond length in S2? Explain.

Apply molecular orbital theory to predict if each molecule or ion
exists in a relatively stable form.
a. C22+
b. Li2
c. Be22+
d. Li22-

MO Theory: Bond Order Concept 2

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1. Intro to General Chemistry

2. Atoms & Elements

3. Chemical Reactions

4. BONUS: Lab Techniques and Procedures

5. BONUS: Mathematical Operations and Functions

6. Chemical Quantities & Aqueous Reactions

7. Gases

8. Thermochemistry

9. Quantum Mechanics

10. Periodic Properties of the Elements

11. Bonding & Molecular Structure

13. Liquids, Solids & Intermolecular Forces

14. Solutions

15. Chemical Kinetics

16. Chemical Equilibrium

17. Acid and Base Equilibrium

18. Aqueous Equilibrium

19. Chemical Thermodynamics

20. Electrochemistry

21. Nuclear Chemistry

22. Organic Chemistry

23. Chemistry of the Nonmetals

24. Transition Metals and Coordination Compounds

[BOOK CHEM] Clutch Version

MO Theory: Homonuclear Diatomic Molecules

Homonuclear Diatomic Molecules are composed of 2 identical elements bonded together.

Equatorial and Axial Positions

Covalent compounds with 5 or 6&nbsp;electron groups have equatorial and axial positions for surrounding elements

MO Theory: Heteronuclear Diatomic Molecules

Heteronuclear Diatomic Molecules are composed of 2 different elements bonded together.

Bond Angles

Bond Angles&nbsp;result from surrounding elements and lone pairs around the central element positioning themselves at an optimal distance.&nbsp;

Molecular Orbital Theory

Molecular Orbital Theory&nbsp;allows us to predict the distribution of electrons within a molecule.&nbsp;

Molecular Orbital Diagrams can be used to determine the bond order of a molecule.

Electron Geometry

The simplest system for geometrical shapes that focuses on the number of electron groups around the central atom.

Hybridization

How would an&nbsp;s orbital combine with a p orbital to form a covalent bond? The answer is through hybridization where a hybrid&nbsp;oribital is formed.&nbsp;

Molecular Geometry

True Shape of Molecules that takes into account differences&nbsp;in repulsion between lone pairs and surrounding elements.