Timeline — History of the Periodic Table — Notes

Year	Scientist(s)	Event
1817	Johann Döbereiner	Law of Triads — groups of 3 elements with middle element's atomic mass ≈ average of outer two (e.g., Li, Na, K)
1865	John Newlands	Law of Octaves — every 8th element has similar properties; limited to Ca; dismissed by contemporaries
1869	Dmitri Mendeleev & Lothar Meyer	Mendeleev's Periodic Table — elements arranged by increasing atomic mass; predicted undiscovered elements (Eka-boron = Sc, Eka-aluminium = Ga, Eka-silicon = Ge)
1870	Lothar Meyer	Published his version simultaneously (independently); focused on physical properties
1894–1898	William Ramsay & Lord Rayleigh	Discovery of noble gases (He, Ne, Ar, Kr, Xe); added Group 18 to the table
1913	Henry Moseley	Moseley's Experiment (X-ray spectra) — atomic number is the fundamental ordering property; replaces atomic mass as the basis → Modern Periodic Law
1945	Glenn Seaborg	Proposed placement of actinoids as a separate f-block series; revised periodic table shape to the long form used today
1951	IUPAC	Standardized nomenclature of transuranium elements
2016	IUPAC	Officially named elements 113 (Nihonium, Nh), 115 (Moscovium, Mc), 117 (Tennessine, Ts), 118 (Oganesson, Og); completed Period 7