During the nineteenth century, chemistsbegan to categorize the elementsaccording to similarities in theirphysical and chemical properties. Theend result of these studies was ourmodern periodic table.
Johann Dobereiner1780 - 1849Model of triadsIn 1829, he classified some elements into groups ofthree, which he called triads.The elements in a triad had similar chemicalproperties and orderly physical properties.(ex. Cl, Br, I andCa, Sr, Ba)
John Newlands1838 - 1898Law of OctavesIn 1863, he suggested that elements be arranged in“octaves” because he noticed (after arranging theelements in order of increasing atomic mass) thatcertain properties repeated every 8th element.
John Newlands1838 - 1898 Law of OctavesNewlands claim to see a repeating pattern was metwith savage ridicule on its announcement. Hisclassification of the elements, he was told, was asarbitrary as putting them in alphabetical order andhis paper was rejected for publication by theChemical Society.
John Newlands1838 - 1898 Law of OctavesHis law of octaves failed beyond the elementcalcium.Would his law of octaves work today withthe first 20 elements?
Dmitri Mendeleev1834 - 1907In 1869 he published a table of theelements organized by increasing atomicmass.
Lothar Meyer1830 - 1895At the same time, he published his own table ofthe elements organized by increasing atomic mass.
• Both Mendeleev and Meyer arranged theelements in order of increasing atomic mass.• Both left vacant spaces where unknown elementsshould fit.So why is Mendeleev called the “father of themodern periodic table” and not Meyer, or both?
• stated that if the atomic weight of an elementcaused it to be placed in the wrong group, thenthe weight must be wrong. (He corrected theatomic masses of Be, In, and U)• was so confident in his table that he used it topredict the physical properties of three elementsthat were yet unknown.Mendeleev...
After the discovery of these unknownelements between 1874 and 1885, and the factthat Mendeleev’s predictions for Sc, Ga, andGe were amazingly close to the actualvalues, his table was generally accepted.
However, in spite of Mendeleev’s great achievement,problems arose when new elements were discoveredand more accurate atomic weights determined. Bylooking at our modern periodic table, can youidentify what problems might have caused chemistsa headache?Ar and KCo and NiTe and ITh and Pa
Henry Moseley1887 - 1915In 1913, through his work with X-rays, hedetermined the actual nuclear charge (atomicnumber) of the elements*. He rearranged theelements in order of increasing atomic number.*“There is in the atom a fundamentalquantity which increases by regular stepsas we pass from each element to the next.This quantity can only be the charge onthe central positive nucleus.”
Henry MoseleyHis research was halted when the Britishgovernment sent him to serve as a foot soldier inWWI. He was killed in the fighting in Gallipoli bya sniper’s bullet, at the age of 28. Because of this loss,the British government later restricted its scientiststo noncombatant duties during WWII.
Glenn T. SeaborgAfter co-discovering 10 new elements, in 1944 hemoved 14 elements out of the main body of theperiodic table to their current location below theLanthanide series. These became knownas the Actinide series.1912 - 1999
Glenn T. SeaborgHe is the only person to have an element namedafter him while still alive.1912 - 1999"This is the greatest honor everbestowed upon me - even better, I think,thanwinning the Nobel Prize."