Chemistry Lecture 17 Flashcards

Questions and Answers

Is F2 paramagnetic or diamagnetic?

Diamagnetic (correct)

Paramagnetic

Is O2 paramagnetic or diamagnetic?

Paramagnetic (correct)

Diamagnetic

Is N2 diamagnetic or paramagnetic?

Diamagnetic (correct)

Paramagnetic

Is C2 diamagnetic or paramagnetic?

Diamagnetic Signup and view all the answers

Is B2 diamagnetic or paramagnetic?

Paramagnetic Signup and view all the answers

Why do the energy levels of sigma 2p and pi 2p switch for C2 and B2?

Because the energy of the pi orbital remains essentially constant while the energy of the sigma orbital increases in energy moving from F2 to B2. Signup and view all the answers

What is orbital mixing (hybridization)?

When an orbital can interact with more than a pair of orbitals to create molecular orbitals with varying degrees of Pz and s character. Signup and view all the answers

What is the important result of orbital mixing?

The sigma 2p orbital no longer has pure p character and its energy may rise above that of the pi 2p. Signup and view all the answers

What is the trend for the 2s-2p gap across the period?

The gap increases. Signup and view all the answers

For what elements is orbital mixing negligible?

O2 and F2. Signup and view all the answers

For what elements is orbital mixing important?

N2, C2, and B2. Signup and view all the answers

What is the difference when constructing a heteronuclear diatomic molecule's orbital diagram and a homonuclear diatomic molecule's orbital diagram?

The energies of the atomic orbitals in A and B are different. Signup and view all the answers

For the MO diagram of a heteronuclear diatomic, which element's atomic orbitals are lower in energy?

The more electronegative element. Signup and view all the answers

Why is the bond polar in the bonding orbital?

Because the electron pair in the bonding orbital is not shared equally. Signup and view all the answers

What is the bond order of HF?

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What is the bond order of CO?

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Why are not all combinations of atomic orbitals not possible?

Because the molecular orbitals must possess a symmetry that is compatible with the symmetry of the other molecule. Signup and view all the answers

What are symmetry adapted orbitals?

Two combinations: in-phase and out-of-phase. Signup and view all the answers

Which orbitals on oxygen can interact with the in-phase combination of hydrogen orbitals in H2O?

The 2s and 2pz. Signup and view all the answers

Which orbitals on oxygen can interact with the out-of-phase combination of hydrogen orbitals in H2O?

Only 2py. Signup and view all the answers

Which hydrogen orbitals does 2px interact with?

None; it remains non-bonding. Signup and view all the answers

How many molecular orbits result from the in-phase combination of hydrogen atoms?

Three (bonding, non-bonding, and anti-bonding). Signup and view all the answers

How many molecular orbits result from the out-of-phase combination of hydrogen atoms?

Two (bonding and antibonding). Signup and view all the answers

What are the lone pairs for the molecular orbitals of H2O?

A pure p orbital and an sp^n hybrid. Signup and view all the answers

How do we use MO theory to predict the structures of molecules?

The MO diagrams are calculated for all possible structures and the structure with the lowest energy is the structure of the molecule. Signup and view all the answers

Why is water bent?

Bending allows the 2pz orbital to interact with the in-phase combination of hydrogen orbitals, stabilizing it. Signup and view all the answers

Study Notes

Molecular Properties

F2 is diamagnetic.
O2 is paramagnetic.
N2 is diamagnetic.
C2 is diamagnetic, although molecular orbital diagrams indicate it can be paramagnetic.
B2 is paramagnetic, despite appearing diamagnetic in molecular orbital diagrams.

Energy Levels and Orbital Mixing

The energy levels of the sigma 2p and pi 2p orbitals switch in C2 and B2 due to the pi orbital's energy being relatively constant while the sigma orbital energy increases from F2 to B2, crossing above pi at N2.
Orbital mixing, or hybridization, occurs when an orbital interacts with multiple orbitals, creating molecular orbitals with varying Pz and s character.
Important outcome of orbital mixing is that the sigma 2p orbital no longer maintains pure p character, with its energy possibly exceeding that of pi 2p.

Trends and Molecular Diagrams

The gap between 2s and 2p orbitals increases across the period due to more effective penetration of the 2s orbital compared to the 2p.
Orbital mixing is negligible for O2 and F2, but is significant for N2, C2, and B2.
Heteronuclear diatomic molecular orbital diagrams differ from homonuclear diagrams as they account for different energy levels of the atomic orbitals of the constituent elements.

Bond Characteristics

In a heteronuclear diatomic molecule, the more electronegative element's atomic orbitals are lower in energy.
Bonds are polar in bonding orbitals when electron pairs are unequally shared, leading the molecular orbitals to be more similar to the atomic orbitals of the closest energy.
The bond order for HF is 1 and for CO it is 3.

Molecular Orbitals Compatibility

Not all combinations of atomic orbitals are feasible; molecular orbitals must maintain symmetries compatible with other molecules' symmetries.
Symmetry adapted orbitals exist in two forms: in-phase and out-of-phase combinations.

Water Molecular Structure

2Px on oxygen does not interact with hydrogen orbitals and remains nonbonding due to cancelation of in-phase and out-of-phase overlaps.
In-phase combinations of hydrogen atoms result in three molecular orbits (bonding, nonbonding, and anti-bonding).
Out-of-phase combinations yield two molecular orbits (bonding and anti-bonding).
Hydrogen fluoride (H2O) has lone pairs represented as a pure P orbital and an sp^n hybrid.

MO Theory Application

Molecular orbital (MO) theory helps predict molecular structures by comparing calculated MO diagrams; the structure with the lowest energy is favored.
Water exhibits a bent structure due to the interaction of the Pz orbital with the in-phase combination of hydrogen orbitals, providing additional stability not achievable in a linear configuration.

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Description

These flashcards cover the concepts of paramagnetism and diamagnetism as they relate to different molecular species. Test your knowledge by determining whether F2, O2, N2, C2, and B2 are paramagnetic or diamagnetic based on their molecular orbital diagrams. A great study aid for understanding molecular electronics and magnetic properties in chemistry.