John Newlands (1837-1898)  an English chemist  tried to classify elements by their atomic weight, and noticed a repeating pattern of every eight elements.  He used the analog with music and called it the Law of Octaves.   However, determining atomic weights was based on comparing other elements to the lightest element which was hydrogen and given a value of 1.00.  Some of the elements were inaccurately given values.  He was ridiculed by other chemists who felt the table he created was not reliable.  He could not get his papers published and returned as chief chemist in a sugar factory and later opened a chemical business with his brother.        

Dmitri Mendeleev  (1834-1907) rose from very poor beginnings to a position of a renowned Russian chemist in the 19th century.  He wrote down information on each element on cards.  He noticed that the atomic weight (we now refer to as the atomic mass) could be used to rank the elements from lightest to heaviest.    Mendeleev noticed that if you looked at their chemical properties there was a periodicity of properties, especially if you left spaces where the properties did not fit.  He stressed that there was order to the elements, and that this order would help predict elements not yet discovered.   He used the calculated atomic weights of the time (which had improved since Newlands) and used them to put the elements in a table.  Mendeleev predicted the properties of aluminum, boron, and silicon.   Gallium, scandium, and germanium were also found to fit his predictions.  There were some discrepancies, but the format of the table is the basis of today’s periodic table.   Mendeleev also noted errors in the atomic weight of some elements. Mendeleev’s table as published in 1869, had many gaps and questions.   If you think it does not look like today’s table, read the horizontal lines, and notice they are today’s periodic table’s Groups I-VIII.