jan 1, 1864 - John Newlands and his "Law of Octaves"
In 1864, John Newlands groups the 56 known elements into 11 groups, based on their properties.
During his grouping, Newlands noticed a pattern that occured with these elements, such as the fact that they had similar pairs for every 8 elements.
Unfortunately, Newlands faced ridicule for his discoveries as the music and science communities at the time were segregated and both communities didn't want to associate with one another.
Added to timeline:
History of the Periodic table