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| | The Periodic Table — FactMonster.com |
 | | Although some elements, such as gold and iron, have been known to humans since prehistoric times, it wasn't until the 17th century that the first scientific discovery of an element (phosphorus) was made. |
 | | Only 12 elements were known prior to 1700, but as more and more elements were discovered—by 1900 there were more than 80—scientists tried to find a way to organize them systematically, according to their physical and chemical properties. |
 | | De Chancourtois was the first to notice the periodicity of the elements, that is, when the elements were arranged according to their atomic weights, similar elements seemed to occur at regular intervals. |
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