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He classified some elements into groups of three (triads). The elements in a triad had similar chemical properties and orderly physical properties - model of triads
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In 1863, he suggested that elements be arranged in octaves because he noticed (after ordering the elements in increasing atomic mass) that certain properties repeated every 8th element - Law of octaves.
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In 1869, he published a table of the elements organised by increasing atomic mass.
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In 1869, he also published a table of the elements organised by increasing atomic mass.
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In 1913, through his work with X-rays, he determined the actual nuclear charge (atomic number) of the elements. He rearranged the elements in increasing atomic number.
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After co-discovering 10 new elements, in 1994 he moved 14 elements out of the main body of the periodic table to the Actinoids series. He also had an element named after him while he was still alive.
