[zeshkani]

hi there

can anybody explain Molecular symmetry, like the n folding, rotation operation, and improper rotation, inversion,  i mean the teacher did cover it, but it was so difficult to understand, and he went through it to fast.
anybody that can help or advise something, would be great thanks

[Bakegaku]

We'd like to help you, but something as vague as that will be difficult to answer.  Try searching for molecular symmetry on google, then coming back with some specific questions on the matter.

7&8:
http://www.chemicalforums.com/index.php?page=forumrules

[enahs]

I would start here and review with this:
http://www.ch.ic.ac.uk/local/symmetry/

Then ask again with what you need more clarification with.

[Goldfingers]

A good way to learn to identify symmetry elements in a point group is to construct some complexes with a chemical model set, find out what point group they belong to to and look it up in a character table (can be found in any inorganic chemistry book, atkins phys. chem etc). It will give you all the members of that symmetry group, i.e. all elements of the form Cn, Sn, I, sigma h's, v's and d's.

Start with simple molecules like water, ammonium, benzene. (C2v, C3v and D6h). Build, them with a chemical model set and play around with them and try to identify the symmetry elements that are in the character table . After you have learnt to identify symmetry elements, build some inorganic octahedral and tetrahedal complexes and try to ligate them in as many ways you can. After a bit of practise, you should be fine.

The basic concepts of proper and improper rotations, mirror planes and centers of symmetry can be found in almost any inorganic or physical chemistry course book. However one book that is ofter used and provides a very simple  step by step guide to chemical group theory is (inexpensive paperback):

Molecular Symmetry and Group Theory : A Programmed Introduction to Chemical Applications, 2nd Edition  by Alan Vincent

The best advanced text book (very valuable as reference), that also provides you with the basic mathematical machinery of group theory, and is the classic in this subject is:

Chemical Applications of Group theory by Albert Cotton, (now in 3rd edition i believe). You can probably get hold of it from your university library.

Many people have a very hard time to learn molecular symmetry from a blackboard, not only you, so practise with real 3d structures help a lot. As previously said, you can probably get the answers to your questions from the above links, dont hesitate to ask if you need clarifacation on any concepts.