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400
Atom proposal
400bc, Democritus and Leucippus propose the idea of the atom, an indivisible particle that all matter is made of. (please excuse inaccuracy of title date. The website will not let me enter 'bc') -
Antoine Lavoisier
1778
Antoine Lavoisier wrote the first extensive list of elements containing 33 elements & distinguished between metals and non-metals -
Law of Triads
In 1817 Johann Dobereiner noticed that the atomic weight of strontium fell midway between the weights of calcium and barium, elements possessing similar chemical properties. In 1829, after discovering the halogen triad composed of chlorine, bromine, and iodine and the alkali metal triad of lithium, sodium and potassium he proposed that nature contained triads of elements the middle element had properties that were an average of the other two members when ordered by the atomic weight (the Law of -
Alexandre-Emile Béguyer de Chancourtois
French geologist Alexandre-Emile Béguyer de Chancourtois listed the elements on paper tape and wound them, spiral like, around a cylinder. Certain ‘threes’ of elements with similar properties came together down the cylinder. He called his model the ‘telluric screw’. -
John Newlands
Newlands arranged the 62 known elements in order of increasing atomic weights, noted that after interval of eight elements similar physical/chemical properties reappeared. Newlands was the first to formulate the concept of periodicity in the properties of the chemical elements. -
Mendeleev
In 1869, Russian chemist Dmitri Mendeleev produced a periodic table based on atomic weights but arranged ‘periodically’.He found that the table showed similarities in an entire networks of vertical, horizontal, and diagonal relationships. Elements with similar properties appeared under each other. Gaps were left for yet to be discovered elements. He was the biggest break through of the discovery of the periodic table. -
Noble gases
In 1895 Lord Rayleigh reported the discovery of a new gaseous element named argon which proved to be chemically inert. This element did not fit any of the known periodic groups. -
Noble gases continued
In 1898, William Ramsey suggested that argon be placed into the periodic table between chlorine and potassium in a family with helium, despite the fact that argon's atomic weight was greater than that of potassium. This group was termed the "zero" group due to the zero valency of the elements. -
Moseley
Moseley arranged the elements according to increasing atomic numbers and not atomic masses, some of the inconsistencies associated with Mendeleev's table were eliminated. The modern periodic table is based on Moseley's Periodic Law (atomic numbers). -
Glenn Seaborg
In 1940, Glenn Seaborg artificially produced heavy mass elements such as neptunium. These new elements were part of a new block of the periodic table called ‘actinides’. This was the last major change to the periodic table.
