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.