Chem 109  General Chemistry  Spring 2015
Lecture Notes 34: 22 April
Molecular Orbital Model of Bonding, cont.
Molecular Orbital Energy Levels and Bonding in Diatomic Homonuclear Molecules, Li_{2}Ne_{2}

Li_{2} 
Be_{2} 
B_{2} 
C_{2} 
N_{2} 
O_{2} 
F_{2} 
Ne_{2} 
sigma_{2p}* 








pi_{2p}* 








sigma_{2p} 








pi_{2p} 








sigma_{2s}* 








sigma_{2s} 








Bonds 
1 
0 
1 
2 
3 
2 
1 
0 
Paramagnetic 
N 
N 
Y 
N 
N 
Y 
N 
N 
Homonuclear molecules, cont.
Last time we were looking at the various homonuclear molecules of Period 2. One of the problems of the model we used was that we would predict the sigma 2p orbital would be at a lowere enernergy than the pi 2p orbitals, but in the chartt above that is not the case. What is going on? The problem is that if we look at the orbitals one at a time (sigma separately from p) then the morsymmetrical sigma orbital will be lower in energy, but if we mix all three p orbitals in making our new molecular orbitals, then the energies shift to give us the combinations in the table. How do we knwo this is correct? Look at B_{2} and O_{2} where the Table predicts the molecules will be paramagnetic (they have unpaired electrons) where as the model with the sigma 2p at a lowere energy would give nonparamagnetic molecules.
Heteronuclear Molecules:
We've looked at homonuclear molecules where the initial orbital energies are identical, what about the more complex situation where two different atoms combine?
 An example with close energies is NO.
 Here we can use the homonuclear model as a close approximation. [text Fig 9.40]
 Note the correct prediction of paramagnetism and bonding.
 The hybrid orbital theory fails in these predictions.
Molecular Orbital Energy Levels in NO
sigma_{2p}* 

pi_{2p}* 

sigma_{2p} 

pi_{2p} 

sigma_{2s}* 

sigma_{2s} 

Bonds 
2.5 
 A classic example, because of its extremity is HF.
Here we have to use a different model. Here it is no reasonable to use the homonuclear model because the electronegativities of the two atoms are so different. Thde electrons will not redistribute in an even sharing. Thus we assume most of the electron density will end up with F and therefore we will assume just the one p orbital of F will be involved.

H atom

HF molecule

F atom

E


sigma* 

1s

















2p 


sigma 

 To simplify our modeling we can assume that we use a single orbital from fluorine, the outermost porbital.
 Again, to simplify our thought processes, let's assume localized electrons for the FILLED s and the two filled p orbitals on F.
 We will then form a sigma orbital between the halffilled p orbital of F and the 1s orbital of H.
 Keep in mind that Hydrogen has no p orbitals, no pi orbitals will be formed, the sigma bond is the only one that makes sense  a similar rationale will also work for atoms such as Li and Be.
 To form the orbital we next need to think about the energy levels of the two participating orbitals.
 From electronegativities we recall that fluorine has a greater attraction for electrons than hydrogen, so we get the diagram seen below. [text Fig 9.42]
 We can also see this in the distribution of electrons in the sigma orbital. [text Fig 9.43]
Simple Models for Complex Molecules: Benzene texts
Models and theories:
 Theory  an explanation of observations consistent with results of experiments etc.
 The theory is a "model of reality"
 Note we also use models which are not intended to represent reality, but rather are used to solve particular problems within a defined "universe" which may mimic the behavior of a restricted subset of "reality."
Making molecular orbital theory work for larger molecules.
 Start with "mechanics"  model based on solid balls and springs. Gives approximate geometries and bond lengths, based on classical physics.
 Tune up with varying degrees of sophistication using different quantum models optimized to solve different problems. Each model describes a slightly different "universe" which corresponds more or less well to our own. Must chose best model to solve a particular problem.
Solutions (Chapter 11)
Solution Concentrationsa Review & Some New Stuff.
Solutions —General Information
Solutions: a solution occurs when one chemical is completely dissolved or dispersed in another. We most commonly think of solutions as being liquid, but solid solutions also occur, such as the various metal alloys like steel, brass and bronze.
In a solution the substance present in highest concentration is considered to be the solvent, while components in lesser amounts are considered to be solutes. If you dissolve a sugar cube in water you get a sugar solution, where water is the solvent, and sugar is the solute.
FYI
Example:
 What is the solvent in 80 proof rum: 80 proof = 40% alcohol in water, so water is the solvent.
 What is the solvent in 151 proof rum: 151 proof = 75.5% alcohol in water, so alcohol is the solvent.

Solubility
All gases are completely soluble in each other.
Liquid solutions
"Like dissolves like."
 Solvation  the process of surrounding a solute with solvent molecules).
 Hydration (solvation with water).
 Saturation  the maximum amount of solute which can be in solution in equilibria with its pure state.
 Supersaturation  dissolving solute in excess of saturation  unstable to the addition of solute (carbonated beverages, honey, etc.) Video
© R A Paselk
Last modified 20 April 2015