Before revision: Dmitri Mendeleev had 63 elements in his periodic table of 1869. upDmitri Mendeleev, 1897 From Boyle until the early 20th century, an element was defined as a pure substance that could not be decomposed into any simpler substance.
After revision: Dmitri Mendeleev had 63 elements in his periodic table of 1869. upDmitri Mendeleev, 1897 From Boyle until the early 20th century, an element was defined as a pure substance that cannot be decomposed into any simpler substance.