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400 BCE
Democritus
Democritus was an ancient Greek philosopher. He was curious about what "matter" was. He believed that matter could be cut into increasingly smaller pieces. And, according to Democritus, atoms were so tiny that they couldn't be seen with the naked eye, but they could be formed into various shapes and sizes. -
Period: 300 BCE to
Aristotle
Aristotle believed that all elements were made up of four elements: fire, water, earth, and air. He also believed that matter had only four properties: dry, cold, hot, and wet. -
John Dalton
Dalton said that atoms are the building blocks of all matter. Atoms are invisible, "uncuttable," and indestructible. And all atoms of the same element are identical, while atoms of different elements are distinct. -
Dmitri Mendeleev
Mendeleev was the first person to create and arrange the periodic table of elements. He made the table in 1869, leaving gaps for unknown elements he wasn't sure about. -
J. J. Thomson
Thomson was the first to suggest that an atom is made out of smaller particles since he discovered the negative charge of an electron. From this, he proposed a model called the "Plum Pudding Model," which is supposed to demonstrate the atom. -
Ernest Rutherford
Rutherford stated that the atom has so much open space because the nucleus is so much smaller than the atom. He then concluded that the negatively charged particles are scattered at a distance outside. -
Robert Andrews Millikan
Robert Millikan was the first to precisely determine the magnitude of an electron's charge in 1910. Millikan demonstrated that the charge of the various drops of oil was always a multiple of a precisely determined charge—the charge of the electron. -
Niels Bohr
Bohr's contributions included developing the idea that an atom's mass is mostly contained in its nucleus. Bohr also proposed that electrons move in definite orbits around the nucleus, similar to how planets orbit the sun.
