history of the modern periodic table

• During the nineteenth century, chemists began to categorize the elements according to similarities in their physical and chemical properties.
• The end result of these studies was our modern periodic table.
• Johann Dobereiner classified some elements into groups of three, which he called triads, in 1829.
• The elements in a triad had similar chemical properties and orderly physical properties.
• John Newlands suggested that elements be arranged in “octaves” because he noticed that certain properties repeated every 8th element in 1863.
• John Newlands' claim to see a repeating pattern was met with savage ridicule on its announcement.
• Dmitri Mendeleev published a table of the elements organized by increasing atomic mass in 1869.
• Lothar Meyer published his own table of the elements organized by increasing atomic mass at the same time as Dmitri Mendeleev.
• Both Mendeleev and Meyer arranged the elements in order of increasing atomic mass and left vacant spaces where unknown elements should fit.
• Dmitri Mendeleev was confident in his table that he used it to predict the physical properties of three elements that were yet unknown.
• After the discovery of these unknown elements between 1874 and 1885, and the fact that Mendeleev’s predictions for Sc, Ga, and Ge were amazingly close to the actual values, his table was generally accepted.
• Problems arose when new elements were discovered and more accurate atomic weights determined.
• In 1913, through his work with X-rays, Henry Moseley determined the actual nuclear charge (atomic number) of the elements.
• Henry Moseley rearranged the elements in order of increasing atomic number.
• Henry Moseley’s research was halted when the British government sent him to serve as a foot soldier in WWI.
• Henry Moseley was killed in the fighting in Gallipoli by a sniper’s bullet, at the age of 28.
• Glenn T. Seaborg moved 14 elements out of the main body of the periodic table to their current location below the Lanthanide series in 1944, these became known as the Actinide series.
• Glenn T. Seaborg is the only person to have an element named after him while still alive.
• Mendeleev stated that if the atomic weight of an element caused it to be placed in the wrong group, then the weight must be wrong.
• Dmitri Mendeleev corrected the atomic masses of Be, In, and U.
• Dmitri Mendeleev used his table to predict the physical properties of three elements that were yet unknown.
• After the discovery of these unknown elements between 1874 and 1885, and the fact that Mendeleev’s predictions for Sc, Ga, and Ge were amazingly close to the actual values, his table was generally accepted.