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The Law of Triads
Johann Dobereiner noticed that the atomic weight of strontium fell midway between the weights of calcium and barium, elements possessing similar chemical properties. He also noticed the alkali metal triad (Li/Na/K) and the halogen triad (Cl/Br/I). In 1829 Dobereiner proposed the Law of Triads: Middle element in the triad had atomic weight that was the average of the other two members. -
The fist Periodic Table
Alexandre Beguyer de Chancourtois, a geologist, first recognized periodicity in the physical properties of the elements. He published a list of all the known elements that constructed as a helical graph wrapped around a cylinder; elements with similar properties occupied positions on the same vertical line of cylinder. It also included some ions and compounds. -
The Law of Octaves
John Newlands arranged the 62 known elements in order of increasing atomic weights, and noticed that after interval of eight elements similar physical/chemical properties reappeared. He was the first to formulate the concept of periodicity in the properties of the chemical elements. He wrote a paper proposing the Law of Octaves, that elements exhibit similar behavior to the eighth element following. -
Father of the Periodic Table
Mendeleev arranged the elements by atomic weights and properties. His periodic table of 1869 contained 17 columns with two partial periods of seven elements each (Li-F & Na-Cl) followed by two nearly complete periods (K-Br & Rb-I). -
17-Group with 8 Columns Periodic Table
Mendeleev revised the 17-group table with eight columns (the eighth group consisted of transition elements). This table exhibited similarities not only in small units such as the triads, but showed similarities in an entire network of vertical, horizontal, and diagonal relationships. -
Noble Gases
Lord Rayleigh and William Ramsey greatly enhanced the periodic table by discovering the "inert gases." Rayleigh reported the discovery of a new gaseous element named argon. This element was chemically inert and did not fit any of the known periodic groups. Ramsey followed by discovering the remainder of the inert gases and positioning them in the periodic table. -
Moseley's Periodic Law
Henry Moseley subjected known elements to x-rays. He was able to derive the relationship between x-ray frequency and number of protons. He 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. -
Modern Periodic Table
Glenn Seaborg discovered transuranium elements 94 to 102 and reconfigured the periodic table by placing the lanthanide/actinide series at the bottom of the table. He made the last Major change in the Periodic table.
