The Joy of Science
The Periodic Table of the Elements
TV-G
S1 • E20 30mLong before Bohr, chemists realized that there are many kinds of atoms—the chemical elements. Elements cannot be broken down into other substances by any ordinary physical or chemical means. In 1869, Dmitri Mendeleev used observed similarities to draw up a periodic table of 63 chemical elements. Subsequent discoveries have lengthened the table but not altered its basic form.