Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 110	General Chemistry	Summer 2006
Lecture Notes::Lec 16_21 June	© R. Paselk 2006
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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).

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

The Representative Elements

Hydrogen:

• 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