Chemicool
Chemicool.com: Silicon
This site from Chemicool.com provides a very through set of tables summarizing many physical and chemical properties of silicon.
Amethyst Galleries
Amethyst Galleries: Mineral Gallery: Minerals by Class
This is a detailed reference list of minerals and mineral classes.
Environmental Chemistry
Environmental chemistry.com: Periodic Table Nitrogen
An exceptionally detailed and helpful resource for all sorts of information on nitrogen.
Environmental Chemistry
Environmental chemistry.com: Periodic Table Oxygen
An incredibly thorough site with a wealth of basic information on oxygen. Highly recommended.
Environmental Chemistry
Environmental chemistry.com: Periodic Table Arsenic
This is as thorough as site as you will find for information of all kinds on arsenic. Very helpful.
Environmental Chemistry
Environmental chemistry.com: Periodic Table Selenium
Excellent resource for just about anything you could ever want to know about selenium.
Environmental Chemistry
Environmental chemistry.com: Periodic Table Silicon
Very thorough site with just about everything you could ever need to know about silicon.