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7 horizontal columns on the periodic table of elements to order things such as the reactivity of the elements.

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The properties of elements Change with a pattern. Starting from the left, you have reactive metals. Further down you have relatively unreactive metals. further to the right, there are metalloids. Finally on the far right, there are non reactive gases.

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Russian chemist Dimitri Mendeleev created the periodic table in 1869 by arranging chemical elements by atomic mass. He predicted that there were many more to be elements to be found in the future, he even left blank spaces for were they would need to go. Most, if not everything that Dimitri has predicted has come to be, as many recently discovered elements turned out to fit right in those holes.

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What Are Periods & Groups In The Periodic Table? | Properties of Matter | Chemistry | FuseSchool