-
chemist Johann Dobereiner groups elements with similarities.
Calcium (atomic weight 40), strontium (atomic weight 88), and barium (atomic weight 137) possess similar chemical prepares. Dobereiner noticed the atomic weight of strontium fell midway between the weights of calcium and barium. Dobereiner noticed the same pattern for the alkali metal triad (Li/Na/K) and the halogen triad (Cl/Br/I). -
The Law Of Triads
Dobereiner proposed the Law of Triads: Middle element in the triad had atomic weight that was the average of the other two members. Soon other scientists found chemical relationships extended beyond triads. Fluorine was added to Cl/Br/I group; sulfur, oxygen, selenium and tellurium were grouped into a family; nitrogen, phosphorus, arsenic, antimony, and bismuth were classified as another group. -
60 elements known.
By 1860 about 60 elements were known. -
A List Of All Known Elements Published
Alexandre Beguyer de Chancourtois published a list of all the known elements. The list was constructed as a helical graph wrapped around a cylinder--elements with similar properties occupied positions on the same vertical line of cylinder (the list also included some ions and compounds). Using geological terms and published without the diagram, de Chancourtois ideas were completely ignored until the work of Mendeleev. -
The Law Of Octaves
English chemist John Newlands has listed all the known elements in increasing atomic mass. He discovers that every interval of eight has similar properties. the Law of Octaves: Elements exhibit similar behavior to the eighth element following it in the table. -
Mendeleev's First Design
Russian chemist Dimitri Mendeleev proposed arranging 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). -
Mendeleev's Second Design
Mendeleev revised the 17-group table with eight columns. This table showed similarities not only in small units such as the triads, but showed similarities in an entire network of vertical, horizontal, and diagonal relationships. The table contained gaps but Mendeleev predicted the discovery of new elements. -
Inert 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. -
Taking Shape
the periodic table was taking shape with elements were arranged by atomic weight. -
Henry Moseley Subjected Known Elements To X-rays
He was able to derive the relationship between x-ray frequency and number of protons. When 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.
