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350
Aristotle
Aristotle was born in Greece in 384 BC and died in 322 BC. He was not able to comprehend how atoms could stay in eternal motion in a void. He did not carry out any experiments to prove his beliefs that all matter was made up of the four elements, fire, water, earth and air. He also thought that there were four qualities that connected the elements, dryness, hotness, coldness and moistness. (350 BC) -
430
Democritus
Democritus was born in 460 BC and died in 370BC. He adopted the atomic theory from his mentor, Leucippus. He did not use experiments, rather he theorised that matter can only be divided a certain number of times, until the smallest part is evident. He called this ‘atomos’ meaning not to be cut. Democritus stated that the universe is made up of atoms and the void in which they exist and move. His theory stated that all matter consists of invisible particles called atoms. -
430
Democritus
Atoms are indestructible. Atoms are solid but invisible. Atoms are homogenous. Atoms differ in size, shape, mass, position, and arrangement. Solids are made of small, pointy atoms. Liquids are made of large, round atoms. Oils are made of very fine, small atoms that can easily slip past each other. His atomic model had no electrons, neutron or protons.It helped people comprehend the idea of the idea of the atom, and helped other scientists build up the atom into what it is today. (430 BC) -
John Dalton
John Dalton was born in 1766 and died in 1844. He experimented with gas, to come up with his atomic theory that stated that different elements are made up of atoms that differ in size and mass. Atoms of the same element are identical. Atoms of different elements can be identified by their unique atomic number. Atoms of different elements merged are called compounds. -
Joseph John Thomson
J.J Thomson was born in Manchester in 1856, and died in 1940. Thomson performed a series of experiments and discovered the electron. The experiments were designed to study the nature of electric discharge in a high-vacuum cathode-ray tube. He came up with the plum pudding model of the atom, which stated that atoms are made up of a positively charged sphere- like a pudding- and contained negatively charged particles (electrons)- plums. -
Ernest Rutherford
Ernest Rutherford was born in 1879 in New Zealand, and died in Cambridge in 1937. He experimented with Thomson’s plum pudding model. He bombarded gold foil with alpha particles, and found that most of the alpha particles passed straight through the gold foil, and few got deflected. He used this experiment to discover the nucleus, and state that an atom is 99.99% empty space and that the nucleus is positively charged and makes up most of the mass of the atom. -
Max Planck
Max Planck was born in Schleswig in 1858, and died in West Germany in 1947. Planck found the connection between energy and the frequency of radiation by realising that the energy emitted by the resonator could take on values or quanta. The energy for a resonator of frequency v is hv where h is a universal constant, now called Planck's constant. -
Hantaro Nagaoka
Hantaro Nagaoka was born in 1865, and died in 1950. He came up with the saturnian model, stating that the atom is a positively charged sphere surrounded by numerous amounts of negatively charged electrons that are bound by electrostatic forces, similar to Saturn and its rings.There was no experimental proof behind Nagaoka’s theory. -
Henry Moseley
Henry Moseley was born in Weymouth, England in 1887, and died on a Gallipoli battlefield in 1915. Moseley used x-ray tubes to establish the charges on the nuclei of most atoms. He took note that, “The atomic number of an element is equal to the number of protons in the nucleus.” He used this and rearranged the periodic table to be in an order that is related to atomic number rather than atomic mass. -
Niels Bohr
Niels Bohr was born in Denmark in 1885, and died in 1962. Bohr came up with a ‘planetary’ model for the Hydrogen atom.He stated that electrons move around the nucleus in fixed orbits, and have constant energy. When an electron absorbs energy, it moves to a higher orbit. To fall back to its original orbit, it has to release its energy as radiation. -
Louis de Broglie
Louis de Broglie was born in 1892 in France, and died in 1987. He suggested that particles could behave as waves. He thought this because matter and energy were the same thing but had a different appearance. Associated with photons, particles of energy, are wavelengths. For this reason, electrons should also have wavelengths. Two years later an experiment was conducted supporting de Broglie’s theory. -
Werner Heisenberg
Werner Heisenberg was born in 1901, and died in 1976. He discovered that hitting electrons with gamma rays to measure its properties would alter the electron’s behaviour. You could measure the position of an electron or another particle, or its momentum. But the more accurately you measure one, the less precise the other will be. He called this the uncertainty principal. He also found that all electrons that contain photons would change physics and momentum. -
Erwin Schrodinger
Erwin Schrodinger was born in Austria in 1887, and died in Austria in 1961. Schrodinger took Bohr’s thoughts in another direction, developing the probability function for the Hydrogen atom. Probability function describes the cloud-like region that electrons are likely to be located in. -
James Chadwick
James Chadwick was born in Manchester, United Kingdom, in 1891 and died in Cambridge, United Kingdom in 1974. Chadwick discovered the neutron by bombarding beryllium with alpha particles, and letting the radiation that it had emitted, to fall onto paraffin wax. -
James Chadwick
Some suggested the radiation emitted was gamma radiation. Chadwick found that energy and momentum were not conserved in its production. He explained the observations by stating that the radiation was made up of neutral particles that had a similar mass to that of a proton. He named the neutral particle ‘neutron’. -
Lise Meitner
Lise Meitner was born in Austria in 1878, and died in England in 1968. Uranium was bombarded with neutrons and the results were observed. Meitner and a co-worker supposed that when the nucleus of uranium absorbed a neutron, sometimes the nucleus divided into almost equal halves and large amounts of energy were released. Later this process was called nuclear fission. -
Murray Gell-Mann
Murray Gell-Mann was born in 1929, and is still alive. Gell-Mann suggested that all the particles in the nucleus are composed of miniscule parts, holding a fraction of the charge, he called these quarks. He said that every proton and neutron contain three quarks, which are points of energy which are fast moving. His theory was confirmed by a group of scientists, when they bombarded a hydrogen atom with a high-speed stream of electrons and detected the quarks inside.
