1. History of the Periodic Table

2. 19 th Century In the early 19 th century, chemists began to categorize elements according to similarities in their physical and chemical properties. Over time, the modern periodic table was formed.

3. Johann Dobereiner (1829) Classified some elements into groups of 3, which he called triads The elements in the triad had similar chemical properties and orderly physical properties Example: Cl, Br and I, Ca, Sr and Ba Referred to as Model of Triads

4. John Newlands (1863) Suggested that elements be arranged in “octaves” because he noticed (after arranging the elements in order of increasing atomic mass) that certain properties repeated every 8 th element. Law of Octaves

5. Newlands Cont’d Newlands’ claim to see a repeating pattern was ridiculed by his peers. Some thought his classification was as arbitrary as arranging the elements alphabetically and was rejected by the Chemical Society.

6. Mendeleev (1869) Published a version of the table with the elements by increasing atomic mass

7. Lothar Meyer At the same time as Mendeleev, Meyer published his own table with the elements organized by increasing atomic mass.

8. Both Mendeleev and Meyer arranged the elements by increasing atomic mass Both left vacant spaces where unknown elements could fit So why is Mendeleev known as the “Father” of the Modern Periodic table and not Meyer (or not both)?

9. 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. (He corrected the masses of Be, In, and U.) Mendeleev was so confident in his table that he used it to predict the physical properties of three yet unknown elements After the discovery of Sc, Ga, and Ge, it was noted that his predictions were fairly accurate, and thus his table was generally accepted.

10. But… Despite his great achievement, problems arose when new elements were discovered and more accurate atomic weights were determined. If the only pattern was atomic weight, our current table would have issues with the following: Ar and K, Co and Ni, Te and I, Th and Pa

11. Henry Moseley (1913) Through his work with x-rays, he determined the actual nuclear charge (atomic #) of the elements He then rearranged the elements in order of increasing atomic # Killed in Action at age 28 (after this, British scientists were no longer permitted to be in combat roles).

12. Glenn Seaborg After co-discovering 10 new elements, in 1944, he moved elements out of the main body of the table to their current location below the Lanthanide series. They are now named the Actinide series.