20% Correct  - Mendeleev Prize

From wikipedia:

Unknown to Mendeleev, several other scientists had also been working on their own tables of elements. One was John Newlands, who published his Law of Octaves in 1864. However, the lack of spaces for undiscovered elements and the placing of two elements in one box were criticised and his ideas were not accepted. Another was Lothar Meyer, who published a work in 1864, describing 28 elements. Like Newlands, Meyer did not seem to have the idea of using a table to predict new elements. In contrast to Newlands' methodical approach to creating a table, Mendeleev's was almost accidental and emerged gradually.

As a better understanding of atomic weights was developed and better data became available, Mendeleev made for himself the following table:

By adding additional elements following this pattern, he developed his version of the periodic table..

On March 6, 1869, Mendeleev made a formal presentation to the Russian Chemical Society, entitled The Dependence between the Properties of the Atomic Weights of the Elements, which described elements according to both weight and valence. This presentation stated that

1. The elements, if arranged according to their atomic mass, exhibit an apparent periodicity of properties.
2. Elements which are similar as regards to their chemical properties have atomic weights which are either of nearly the same value (e.g., Pt, Ir, Os) or which increase regularly (e.g., K, Rb, Cs).
3. The arrangement of the elements in groups of elements in the order of their atomic weights corresponds to their so-called valencies, as well as, to some extent, to their distinctive chemical properties; as is apparent among other series in that of Li, Be, B, C, N, O, and F.
4. The elements which are the most widely diffused have small atomic weights.
5. The magnitude of the atomic weight determines the character of the element, just as the magnitude of the molecule determines the character of a compound body.
6. We must expect the discovery of many yet unknown elements–for example, two elements, analogous to aluminium and silicon, whose atomic weights would be between 65 and 75.
7. The atomic weight of an element may sometimes be amended by a knowledge of those of its contiguous elements. Thus the atomic weight of tellurium must lie between 123 and 126, and cannot be 128. Here he was wrong as the atomic mass of tellurium (127.6) remains higher than that of iodine (126.9).
8. Certain characteristic properties of elements can be foretold from their atomic weights.

Only a few months after Mendeleev published his periodic table of all known elements (and predicted several new elements to complete the table), Meyer published a virtually identical table. Some people consider Meyer and Mendeleev the co-creators of the periodic table, although most agree that Mendeleev's accurate prediction of the qualities of what he called eka-silicon (germanium), eka-aluminium (gallium) and eka-boron (scandium) qualifies him for deserving the majority of the credit for studies.

As others before him had done, he questioned the accuracy of accepted atomic weights, pointing out that they did not correspond to those predicted by the Periodic Law. He noted that tellurium has a higher atomic weight than iodine, but he placed them in the correct order, assuming that the accepted atomic weights at the time were incorrect. He was puzzled about where to put the known lanthanides, and predicted the existence of another row to the table"