Dmitri Mendeleev

An overall understanding of how the elements are similar to each other and why they exhibit their particular chemical and physical properties was slow in coming. Between 1868 and 1870, in the process of writing his book, The Principles of Chemistry, Mendeleev made a table or chart that listed the known elements due to the increasing order of atomic weights. When Mendeleev organized the table into horizontal rows, a pattern became visible.  However, only if he left blanks in the table. If he did so, elements with similar chemical properties showed at regular intervals. Ever so often in vertical columns on the table. Mendeleev suggested that new elements not yet discovered would be discovered to fill in the blank places. He even went so far as to predict the properties of the missing elements. Although many scientists greeted Mendeleev's first table with disbelief, its predictive value soon became clear. The discovery of gallium in 1875, of scandium in 1879, and of germanium in 1886 supported the original idea of Mendeleev's table. Each of the new elements displayed properties that accorded with those Mendeleev had predicted, based on his realization that elements in the same column have similar chemical properties. Claiming a new element now meant not only identifying its one of a kind chemical properties, but discovering the atom's atomic weight so the element could be fitted into the right place in the periodic table. For radioactive atoms that was a difficult challenge. At first these atoms were isolated only in microscopic quantities. The straightforward way to identify them wasn’t by their chemical properties at all, but by their radiations. Until the radioactive atoms could be sorted out with conventional chemistry. However, some scientists were unwilling to call them new elements.