|Lecture Notes::Lec 16_21 June
© R. Paselk 2006
Molecular Orbitals, cont.
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 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.
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).
- 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)
- 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
The Representative Elements
- Most abundant element in the Universe (over 90% of the atoms in the Universe, 75% by mass, about 25% helium, everything else is trace).
- Rare in Earth's atmosphere because it is too light to be held by Earth's gravity.
- Common in the Earth's crust (includes oceans etc.): 0.9% by mass, but 15% by atom, making it the third most abundant atom after oxygen and silicon.
© R A Paselk
Last modified 21 June 2006