Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 110	General Chemistry	Fall 2003
Lecture Notes::Lec 21_20 October	© R. Paselk 2003
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Molecular Orbitals, cont.

Bond Order = (#bonding electrons - # antibonding electrons)/2. Divide by two to get "classical" two electron bond. Bond order gives a measure of bond strength in units of an electron-pair bond.

• If we look at hydrogen, H₂, both electrons go into the ground state (lowest energy) MO, giving a bond order of one, so H₂ has a single bond.
• If we look at the next possible homonuclear diatomic molecule, He₂, the four electrons will first fill the lowest energy MO, but the next two go into the higher energy, antibonding MO. The bond order is then 0, and theory predicts no bonding and no He₂ molecule.

Lets now go back and and look at the bond orders and bonding of the homonuclear diatomic molecules of the second period. (overhead, text figure 9.39) As we can see in each case the bonding is as predicted from molecular orbital theory.

Molecular Orbital Energy Levels and Bonding in Diatomic Homonuclear Molecules, Li₂-Ne₂

	Li2	Be2	B2	C2	N2		O2	F2	Ne2
s2p*						s2p*
p2p*						p2p*
s2p						p2p
p2p						s2p
s2s*						s2s*
s2s						s2s
Bonds	1	0	1	2	3		2	1	0

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?

The classic example, because of its extremity is HF.

• To simplify our modeling we can assume that we use a single orbital from fluorine, the outermost p-orbital.
  • Again, to simplify our thought processes, let's assume localized electrons for the s and the two filled p orbitals on F.
  • We will then form a sigma orbital between the half-filled p orbital of F and the 1s orbital of H.
• 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. (p 439 in your text)

H atom HF molecule F atom E    s*   1s                                 2p      s

• An example with close energies is NO. Note the correct prediction of paramagnetism and bonding, while the hybrid orbital theory fails in these predictions.

Molecular Orbital Energy Levels in NO s2p*   p2p* s2p p2p s2s* s2s Bonds  2.5

Simple Models for Complex Molecules: Benzene overheads

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 solids 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.

The Chemistry of the Elements

The Periodic Table - A Review

Look at the Periodic Chart on the wall. The pattern arises due to a repetition or periodicity of chemical properties. The vertical columns of the charts are called groups, while the rows are referred to a periods.

Note the numbering of the groups. The numbers from 1 - 18 are the internationally accepted numbers. We will also use the I - VIII "American" numbering system. Note that the "tallest" columns comprise what are referred to as the "representative elements" (IA - VIIIA).

Terms:

• Period: the rows of elements showing a repeating pattern of properties (e.g. Na - Ar).
• Group: a vertical column of elements on the table sharing a family resemblance of properties (e.g. Li - Fr).
• Representative elements: the elements of the s-block and p-block (blue and green on the table below).
• Transition metal elements: the elements of the d-block (yellow in the table below).
• Inner-transition metal elements: The f-block or Lanthanides and Actinides (not shown on the table below)
• Groups:
  • IA = alkali metals;
  • IIA = Alkaline earth metals;
  • VIIA = Halogens (note the generic symbol of X standing for any halogen);
  • VIIIA = Noble gases (older = inert gases).

You should know the terminology above.

Periodic Table of the Elements  IA IIA IIIA IVA VA VIA VIIA VIIIA    H  He Li Be    B C N O F Ne Na Mg IIIB IVB VB VI VIIB VIIIB IB IIB  Al Si P S Cl Ar K Ca Sc  Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr  Rb Sr  Y Zr Nb Mo Tc Ru Rh Pd  Ag  Cd  In Sn Sb Te I  Xe  Cs  Ba Lu  Hf Ta  W Re Os Ir  Pt Au  Hg Tl  Pb Bi Po At Rn

© R A Paselk

Last modified 21 October 2003