Various methods of classifying the elements had been attempted long before Mendeleev. In 1789, the French chemist Antoine Lavoisier published a list of 33 chemical elements, grouping them into gases, metals, nonmetals, and earths. The first recognisable periodic table was published in 1864 by the German chemist Julius Lothar Meyer, who listed the elements in rows or columns in order of atomic weight, starting a new row or column when the characteristics of the elements began to repeat.
The recognition and acceptance of Mendeleev's table (which was published five years after Meyer's first attempt) came from two decisions he made. The first was to leave gaps in the table when it seemed that there was an element that had yet to be discovered. Mendeleev was not the first to do this, but he was the first to use the trends in his periodic table to predict the properties of those missing elements, such as gallium and germanium. The second decision was to occasionally ignore the order suggested by the atomic weights and switch adjacent elements, to better classify them into chemical families. For example, he placed tellurium (atomic weight 127.6) ahead of iodine (atomic weight 126.9).
In 1913, the English physicist Henry Moseley showed that Mendeleev's ordering corresponded to the order of increasing atomic number, which is the number of protons found in the nucleus of the element's atom. In the example given above, the atomic number of tellurium is 52 and that of iodine is 53.
© Haydn Thompson 2017