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Development of the Periodic Table
First step – a method for accurately determining atomic mass
Presented by Sanislaw Cannizzaro at the First International Congress of Chemists – 1860
First recognized method for determining atomic mass
Dimitri Mendeleev
Used atomic mass and properties of elements to organize the first Periodic Table
Used card and organized elements while travelling on the train
Recognized that similarities in chemical properties repeated themselves at regular intervals (i.e. periods)
First table published in 1869.
Exceptions to Mass Order
Observed Tellurium and Iodine should be switch out of mass order based on chemical properties
Also true for Argon/Potassium and Cobalt/Nickel
Mendeleev’s Predictions
Predicted 3 new elements based on table designElements had not been discovered at the time of
publication of the Periodic Table
Henry Mosley
1911 – Found Periodic Table better organized by the number of protons (atomic number)
This organization addressed the Te/I issue.Led to Modern Periodic Law:
The physical and chemical properties of the elements are a periodic function of their atomic number.
Modern Periodic Table arranged by atomic number – Similar elements in same group/family
Noble Gases
1894 – William Strutt and Sir William Ramsay discovered Argon
1898 – Ramsey found Krypton and Xenon1900 – Dorn finds Radon
F Block
Added Lanthanides in early 1900’sThen added ActinidesThese elements do not have group numbers –
are referred to as between Groups 3 and 4.