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In 1829 Johann Dobereiner discovered the halogen triad and the alkali metal triad. The halogen triad was made up of elements chlorine, bromine, and iodine. The alkali metal triad was made up of elements lithium, sodium and potassium.
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In 1863, an English chemist named John Newlands wrote a paper categorizing the known 56 elements into 11 groups.He categorized them based on similar physical properties. Newlands published his version of the periodic table in 1864. He proposed the Law of Octaves. The Law of Octaves says that any given element will display analogous behavior to the eighth element.
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In 1864 a German chemist named Lothar Meyer published a textbook that included an abbreviated version of the periodic table that is used to classify elements. His version of the table was made up of half of the known elements listed in order of atomic weight.
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In 1869, a Russian chemist named Dmitri Mendeleev.He organized his materials in families of the known elements that displayed similar properties. Throughout his time sorting the elements and classifying them Mendeleev noticed a pattern in the properties and atomic weights
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In 1895, Lord Rauleigh discovered a new gaseous element named argon. Argon proved to be chemically inert. This element did not fit into any one of the known periodic groups.
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In 1898, William Ramsey suggested that argon be placed into the periodic table between elements chlorine and potassium and in a family with helium. Argon was placed there despite the fact that arong’s atomic weight was greater that potassium.
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In 1913, Henry Moseley determined the atomic number of each element. Moseley made changes and modified the Periodic Law.When he modified the Periodic Law it put elements tellurium and iodine in the correct order.
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In 1940, Glenn Seaborg artificially produced heavy mass elements such as neptunium. These new elements were part of a new block of the periodic table called ‘actinides’. This was the last major change to the periodic table.
