|MadSci Network: Chemistry|
I believe at the time most elements were characterized by their physical properties and actually weight. So when people were working on arranging the elements atomic numbers were not used. At the time when Mendeleev was working at the atomic numbers were not known or suspected.
A Brief History of the Periodic Table http://www.wou.edu/las/physci/ch412/perhist.htm
Early attempt at arranging elements:
TABLE OF SIMPLE SUBSTANCES -- http://web.lemoyne.edu/~GIUNTA/lavtable.html
On the Relationship of the Properties of the Elements to their Atomic Weights: http://web.lemoyne.edu/~GIUNTA/EA/MENDELEEVann.HTML
Try the links in the MadSci Library for more information on Chemistry.