History of chemistry
An important breakthrough in making sense of the list of known chemical elements (as well as in understanding the internal structure of atoms) was Dmitri Mendeleev's development of the first modern periodic table, or the periodic classification of the elements. Mendeleev, a Russian chemist, felt that there was some type of order to the elements and he spent more than thirteen years of his life collecting data and assembling the concept, initially with the idea of resolving some of the disorder in the field for his students. Mendeleev found that, when all the known chemical elements were arranged in order of increasing atomic weight, the resulting table displayed a recurring pattern, or periodicity, of properties within groups of elements. Mendeleev's law allowed him to build up a systematic periodic table of all the 66 elements then known based on atomic mass, which he published in "Principles of Chemistry" in 1869. His first Periodic Table was compiled on the basis of arranging the elements in ascending order of atomic weight and grouping them by similarity of properties.
William Odling
In the 1860s Odling, like many chemists, was working towards classifying the elements, an effort that would eventually lead to the periodic table of elements. He was intrigued by atomic weights and the periodic occurrence of chemical properties. William Odling and Lothar Meyer drew up tables similar, but with improvements on, Dmitri Mendeleev's original table. Odling drew up a table of elements using repeating units of seven elements, which bears a striking resemblance to Mendeleev's first table. The groups are horizontal, the elements are in order of increasing atomic weight and there are vacant slots for undiscovered ones. In addition, Odling overcame the tellurium-iodine problem and he even managed to get thallium, lead, mercury and platinum in the right groups - something that Mendeleev failed to do at his first attempt.