He created the telluric screw and it shows the atomic weights of the elements on the outside of a cylinder, so that one complete turn added an atomic weight increase of 16, (this was the first known way the periodic elements were grouped).
He was the first person to recognise the periodic trends in the properties of elements, (he listed the elements in order of atomic weight, with elements with the same valency arranged in vertical lines).
He noticed that there were similarities between elements with atomic weights that differed by seven, (Law of Octaves).
He has created the first model of the periodic table that we recognize today
William Ramsay discovered the noble gases and then realized that they represented a new group in the periodic table.
Moseley finally figured why iodine and tellurium in Mendeleev's periodic table needed to be switched around (this was because of the atomic mass and the number).