Dmitri Mendeleev

Dmitri Mendeleev (1834-1907)  is most well known for creating the Periodic Table of the Elements.  He was born in Siberia and later went on to become a professor of chemistry at the University of St. Petersburg.  Because he could not find a good textbook on the subject, he began writing his own.  The textbook only took Mendeleev two years to write (1868-1870), and laid the foundation for modern chemistry.  This may seem fast, but when he was only 27 it took Mendeleev 61 days to write a 500 page book on organic chemistry, simply because he thought Russia needed more chemistry books written in Russian, as many science books were still written in Greek and Latin.

Nothing was known about subatomic particles at this time, but atomic weights were known by scientists.  Mendeleev used this knowledge as well as knowledge of elemental properties to create a simple process for organizing the elements: “I began to look about and write down the elements with their atomic weights and typical properties, analogous elements and like atomic weights on separate cards, and this soon convinced me that the properties of elements are in periodic dependence upon their atomic weights.” 

Although some of Mendeleev's original periodic table was incorrect, he really didn't have much to work with.  Only 60 elements had been discovered at that time, and now we know of over 100.  In addition, he was working only based on atomic mass and observable properties and was still able to correctly place many elements and accurately predict elements that would be discovered later, such as Gallium.  Without Mendeleev, it is difficult to say that we would have chemistry as we know it today, and have made as many advancements based off of this knowledge.

http://www.aip.org/history/curie/periodic.htm

http://www.famousscientists.org/dmitri-mendeleev/

The Royal Society of Chemistry-development of the periodic table