Chemistry Chapter 10- Part 2

Front 1

What is the MO theory?

Back 1

Molecular orbital (MO) theory is a method for determining molecular structure in which electrons are not assigned to individual bonds between atoms, but are treated as moving under the influence of the nuclei in the whole molecule.

They are more stable because they reduce the energy associated with bond formation.

Front 2

Variational Method

Back 2

It is a specific application of a more general quantum-mechanical approximation technique. The energy of a trial function within the Schrodinger equation is minimized.

Front 3

Linear Combination of Atomic Orbitals (LCAOs)

Back 3

Simplest trial function that work reasonably well in molecular orbital theory.

The molecular orbital of this is a weighted linear sum of the valence atomic orbitals of the atoms in the molecule.

Front 4

The Bonding Molecular Orbital ________

Back 4

arises out of constructive interference between the overlapping atomic orbitals because both orbitals have the same phases

Front 5

The Antibonding Molecular Orbital _______

Back 5

arises out of destructive interference between the overlapping atomic orbitals because subtracting one from the other means the two interacting orbitals have opposite phases

Front 6

Name 3 things about antibonding orbitals

Back 6

*Has a node in internuclear energy

*Have higher energies than they do in their respective atomic orbitals and therefore tend to raise the energy of the system

*Less electron density in the internuclear region

Front 7

Name 2 things about bonding orbitals

Back 7

*Increased electron density in the internuclear region

*Lower energy due to chemical bonds forming

Front 8

Bond Order

Back 8

(#bonded-#antibonded)/2

*The higher the bond order the stronger the bond

*Positive Bond Order: bond is stable.

Negative Bond Order: OR ZERO bond order indicates that a bond does not form between the atoms. absence of net stabilization.

Front 9

What is homonuclear diatomic molecules?

Back 9

molecules made up of same two atoms from the 2nd-period that form & have between 2 and 16 valence electrons

Front 10

How to do Molecular Orbital Diagram?

Back 10

Look at Valence Electrons

*Distribute the electrons in the right & left 2s orbitals.

* Distribute the electrons in the 2s orbitals to the bonding orbital first and then the antibonding orbital.

Front 11

Explain Li₂

Back 11

Even though Lithium is normally a metal, we can use MO theory to predict whether or not the Li₂ molecule should exist phase.

Two Valence Electrons occupy a bonding molecular orbital.

We approximate the molecular orbital in Li₂ as linear combinations of the 2s atomic orbitals.

Front 12

Explain Be₂

Back 12

The four valence electrons of Be₂ occupy one bonding MO and one antibonding MO. The bond order is zero and we predict that Be₂ should not be stable, which is again consistent with experimental findings.

Front 13

Back 13

σ2pBonding: The bonding MO has increased electron density toward internuclear region due to constructive interference between the two 2p atomic orbitals.

σ*2pAntibonding: has a node between the 2 nuclei (due to destructive interference between the 2 2p orbitals) and is higher in energy than either of the 2px orbitals.

Front 14

Back 14

*Bonding: the electron density in the bonding molecular orbital is above and below the internuclear axis with a node plane that includes the internuclear axis.

*Antibonding: the antibonding orbital has a additional node between the nuclei (perpendicular to the internuclear axis)

Front 15

Back 15

The energies and names of the bonding and antibonding MOs obtained from the combination of the 2py AOs are identical to those obtained from the combination of the 2pz AOs. Only difference is 90 degree rotation about the internuclear axis.

Front 16

s-p mixing is significant in _____ but not in ____ because ___. The result of the mixing is a ____.

Back 16

*B₂, C₂, & N₂

*O₂, F₂, & Ne₂

* in B,C, &N the energy levels of the atomic orbitals are more spaced then in O,F,&Ne.

*different energy ordering for some of these diatomic molecules.

Front 17

The effects of 2s-2p mixing

Back 17

The degree of mixing between two orbitals dec. w/inc. energy diff. between them. This mixing produces a change in energy ordering for the π2p & σ2p molecular orbitals.

Front 18

*Paramagnetic

*Diamagnetic

Back 18

*has unpaired electrons that generate tiny magnetic fields, which align and interact with external field. (liquid oxygen)

*no unpaired electrons & the magnetic fields caused by electron spin & orbital angular moment tend to cancel each other.

Front 19

Configuration for MO diagram

Back 19

σ*2p

π*2p

σ2p

π2p

σ*2s

σ2s

Front 20

When a molecular orbital is approximated as a linear combination of atomic orbitals of different energies, ____________.

Back 20

the lower-energy atomic orbital makes a greater contribution to the antibonding molecular orbital.

Front 21

for s molecular orbital

Back 21

Front 22

for p molecular orbital

Back 22