Dmitri Mendeleev was an irascible Russian chemist most famous for his work in developing the periodic table of elements. He was the youngest of 17 children and after studying to become a teacher, he took an advanced degree in chemistry in 1857. Mendeleev began his work on the periodic table in 1869 while writing a textbook. It was in this book that he began grouping elements in an ascending order according to their atomic weight (essentially relative atomic mass), noticing that chemically similar elements tended to fall into similar columns. Initially not all of the 63 known elements would fit into Mendeleev's table, and so he altered the atomic weights of indium and beryllium so that they would fit (their initial weights were somewhat inaccurate). In 1871 Mendeleev published a revised table, which ... included gaps for elements that had yet to be discovered.