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Until 1750, scientists had identified only 17 elements. These were mainly metals, such as copper and iron. The rate of discovery increased rapidly as chemists began to investigate materials in a systematic way. As the number of known elements grew, so did the need to organize them into groups based on their properties.
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In 1789, French chemist Antoine Lavoisier (la vwah zee ay) grouped the known elements into categories he called metals, nonmetals, gases, and earths. For the next 80 years, scientists looked for different ways to classify the elements. But none of their systems provided an organizing principle that worked for all the known elements. A Russian chemist and teacher, Dmitri Mendeleev (Duh mee tree Men duh lay uff), would discover such a principle.
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