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350
Aristotle (BC)
Aristotle lived from 384 BC to 322 BC in Greece. He did not believe that atoms could exist or that they could always be in motion. He proposed a theory based on the four elements: fire, water, earth and air. he also believed that there were four qualities to these elements - hotness, coldness, dryness and wetness Aristotle significantly slowed down the atomic theory down by saying it did not exist, but even though the theory is now denied, it was used for nearly two thousand years. -
460
Democritus (BC)
Democritus lived from 460 BC to 370 BC in Greece. He didn't carry out experiments, but proposed hypotheses based on thought and reasoning. Democritus believed: All matter is made up of small indestructible units he called atoms, they are mechanically bound - like a hook and eye, always in motion, indivisible, and that everything is made up of atoms. As he was the first to propose any type of atomic theory, he is the most important, because he planted the idea that led to modern theory. -
Jan 1, 600
Thales (BC)
Thales of Miletus lived from 624 BC to 546 BC in Greece. He proposed the idea that water was the origin and basis of all matter. Although this does not seem to be correct, there is some truth in it that helped the growth of the atomic model. Water is nessecary to life, and the first life forms came from the oceans, so scientists understood his ideas and used them to develop their own atomic theories. Thales's water based theory was not disproved until the 1700’s. -
Isaac Newton
Isaac Newton lived from 4th January 1643 to 31st March 1727 in England. He studied gases and the possibility of atoms existing. Isaac performed many experiments on the structure of light, discovering the behaviour of light and that white light has the same system of colours seen in a rainbow. He proposed the idea of a mechanical universe with small, solid, moving masses. He suggested that atoms are held together with forces, and believed matter is made of solid, massy, indestructible particles. -
Antoine Lavoisier
Antoine Lavoisier lived from August 26th 1743 to May 8th 1794 in France. He studied the composition of chemical compounds, finding that compounds containing more than one element always had the same amount of each element. From this he came up with the Law of Conservation - meaning matter can't be made or destroyed. One of the experiments he used to come up with this law was turning mercury (Hg) and oxygen (O) into mercury oxide (HgO). Lavoisier began the discussion on what an atom was exactly. -
John Dalton
John Dalton lived from September 6th 1766 to July 27th 1844 in England. He experimented the structure of mixed gases. Dalton made many discoveries, such as elements are made of of atoms, atoms from different elements differ from each other, while from the same element are identical, atoms cannot be created or destroyed, and chemical compounds are made when different atoms combine in fixed ratios. Daltons theories gave scientists a way to explain the evidence about atoms. -
Robert Brown
Robert Brown lived from December 21st 1773 to June 10th 1858 in Scotland and England. Botanist Robert Brown performed an experiment where pollen grains were put on water and observed, to find that the pollen grains constanly changed directions in random patterns as if bumping into something invisible. Brown discovered that the explanation to this was that the pollen was hitting particles of water - causing the movement. This movement has been named the Brownian motion, after Robert Brown. -
George J. Stoney
George Johnstone Stoney lived from 15th February 1826 to 5th July 1911 in Ireland and England. He is most famous for his theory that electricity was made of separate negatively charged particles he called electrons. He introduced electrons as the "fundamental unit quantity of electricity". He also made notable contributions to cosmic physics and the theory of gases. These contributions and Stoney's research into electrons assisted the discovery of the particle by J.J. Thomson in 1897. -
Joseph John Thomson
Joseph John “JJ” Thomson lived from December 18th 1856 to August 30th 1940 in England.Thomson discovered that atoms actually were divisible, and made up of even smaller negatively charged particles, which we now know as electrons. This helped him to suggest that an atom is like a plum pudding – the positively charges material being the cake and the electons being the fruit. This was named the Thomson plum pudding model. Thomson is also responsible for inventing the cathode ray tube. -
Hantaro Nagaoka
Hantaro Nakaoka lived from August 15th 1865 to December 11th 1950 in Japan. Nagaoka worked on spectroscopy and created an early but incorrect model of an atom using similarities to Saturn’s rings. He proposed the theories that: The nucleus was very massive (like a very massive planet), and electrons orbiting the nucleus were bound by electrostatic forces (like the rings orbiting Saturn, bound by gravitational forces). Rutherford confirmed both theories, although some details were incorrect. -
Robert Andrews Millikan
Robert Millikan lived from March 22nd 1868 to December 19th 1953 in the United States. Millikan is most famous for measuring the charge of an electron with an oil drop - Oil droplets that were sprayed into a chamber fell through the pinhole into an area between two plates (1 positive, 1 negative). X-rays ionized this middle chamber. Particles that did not capture any electrons were attracted to the positive upper plate and either floated upward or fell more slowly. -
Ernest Rutherford
Ernest Rutherford lived from August 30th 1871 to October 19th 1937 in Britain. (New Zealand born). He is most famous for the gold foil experiment, which helped him to understand the interior of an atom. He discovered that there is a nucleus in the centre of an atom, as well as that an atom is mostly empty space with a large amount of positive charge This discovery lead to the model of the atom known as the Rutherford nuclear model. -
Henry Moseley
Henry Mosely lived from November 23rd 1887 to August 10th 1915 in England. Moseley developed the application of x-ray spectra to study the structure of an atom. He discovered that the number of protons in an element determines its atomic number. This was by measuring the wavelength of the x-rays given off by particular metals, and from that he could find the number of positive charges (protons) in the nucleus of an atom. This was the first concept of the atomic number. -
Niels Bohr
Niels Bohr lived from October 7th 1885 to November 18th 1962 in Denmark. Bohr studied the structure of an atom. He concluded that electrons in the atom don’t spiral into the nucleus, but orbit around it. The more energy they have, the further their orbit is from the nucleus. Bohr proposed a theory that the outer orbit of an atom could hold more electrons than the inner orbit. This is called the Bohr model of the atom. This theory gave us a better understanding of why and how atoms bond. -
James Chadwick
James Chadwick lived from October 20th 1891 to July 24th 1974 in England. Chadwick discovered neutrons. (Neutral particles - no electrical charge - having similar mass to a proton). He found they were different from alpha particles because they repelled electrical forces that are in the nucleus of many atoms. He proved that neutrons - making up about half the mass of an atom, existed.
