Other
Web ref.org: Chlorine
A few well done paragraphs on chlorine, with descriptions of the element and its properties. One nice feature is that any technical term has a link to help you better understand it.
Corrosion Source
Corrosion Source: Manganese
This resource presents a good summary of the history, properties, sources, uses and handling concerns of manganese. Also contains some basic data.
Wikimedia
Wikipedia: Dmitri Mendeleev
This biography on scientist Dmitri Mendeleev details his childhood, gives information on his scientific studies, and discusses his formal presentation made to the Russian Chemical Society in which he detailed his findings on elements.