-
340
Aristotle
In 340 BC, Aristotle questioned Democritus' atomic theory that all atoms varied in shapes and sizes, and instead thought that all matter was made up of air, water, fire, earth, or a combination of these four elements. This model held for about 2000 years, and in fact, hindered the development of the atomic theory because nobody questioned his idea. -
460
Democritus
Democritus lived from 460 - 370 BC. During his lifetime, he, along with his mentor Leucippus created the first atomic theory which stated that atoms are indivisible, indestructible and vary in shape and size. There was no previous model of the atom to contend with. To Democritus, the appearance of an atom varied in shape and size depending on the material it made up. -
Antoine Lavoisier
Antoine Lavoisier is mainly known for his Law of Conservation of Matter, that he made in 1789. It states that mass is neither created nor destroyed in a reaction. He was also the one to discover that oxygen needed to be present for combustion to occur. -
John Dalton
In 1802, John Dalton proposed the Atomic Theory, which helped pave the way for more study on atoms. Daltons model of the atom, however, was still a sphere, but it differed from Democritus's model because he said the reason the elements are pure is becasue the atoms in the element are identcal and the reason elements differed was because their atoms differed in mass. He also believed that all compounds were created from combining different elements with each other. -
Antoine Henri Becquerel
Antoine Henri Becquerel lived from 1852 - 1908. Becquerel did not change the atomic theory, but instead investigated radiation along with Marie and Pierre Curie. Becquerel was working with phosphorescence. He put uranium in the sun to absorb energy and saw that it made an imprint on a photographic plate, but after stopping the experiment for a few days he opened the desk where he put the materials and found that radioactive elements didn't have to absorb energy to release it. -
J. J. Tomson
In 1897, JJ Tomson performed an experiment that helped prove the existance of the electron. In the experiment, he shot electricity through a cathode-ray tube devoid of air. He found that electrons could be deflected with an electrical field. When making his model, he knew there were negatively charged electrons in it, so he thought there must be a sea of positive charge dotted with electrons. -
Ernest Rutherford
In 1911, Ernest Rutherford conducted his famous gold foil experiment to try and prove that there was a very small, dense nucleus in the center of every atom. In the experiment, he shot alpha particles through very thin gold foil, and some were deflected, showing there was a dense nucleus. Rutherford developed his model of a electrons orbitting the nucleus at a fixed distance. The experiment also disproved J.J. Tomsons "plum pudding" model. -
Marie and Pierre Curie
Marie Curie was born in Poland, and Pierre Curie in Paris, France. In 1903 Marie, and Pierre Curie, and Henri Bequerel shared a Nobel Prize for discovering spontaneous radioactivity. As well, in 1911, Marie Curie was awarded another Nobel Prize for her discovery of radium and polonium, other radioactive elements.These discoveries helped give a better understanding of radioactivity for later chemists. -
Henry Moseley
Henri Moseley, born in England in 1887, adapted Mendeleevs version of the Periodic Table by more accurately positioning elements in the Periodic Table because of theirr atomic numbers, numbers that, before, were just thought as arbitrary numbers. He was also able to correctly predict other elements on the Periodic Table. -
Max Planck
Max Planck, a German theoretical physicist, created the quantum theory, that changed the way we think about atomic and subatomic processes. In 1918, he won the Nobel Prize for this. This theory also showed that atoms emit the smallest amount of energy, or quanta. -
Niels Bohr
Niels Bohr lived from 1885 - 1962. He corrected Rutherford's model of the atom because Rutherford's model couldn't explain why different atoms emitted light at different wavelenghts, so, in 1922, Bohr proposed that electrons orbitted the nucleus of an atom at varying energy levels, so they would have different properties. -
Rober Millikan
Robert Millikan won a Nobel Prize in 1923 for his work regarding the first measurement of the charge of an electron (1.60217653 * 10 to the -19th). He measured the charge with the Oil Drop Experiment, where he and his student, Harvey Fletcher, measured the charge of tiny, suspended droplets of oil. -
Erwin Schrodinger
Erwin Schrodinger, an Austrian physicist, elaborated on the previous Bohr model of the atom. Schrodiner's model did not portray the electrons of an atom in exact paths around the nucleus, but in a cloud, the denser the cloud was in a certain area, the higher probability there is of finding an electron there. -
James Chadwick
James Chadwick, 1891 - 1974, worked with Ernest Rutherford in 1932 and discovered the neutron, a neutral subatomic particle in the nucleus of an atom. He did this by repeating a procedure for tracking particle radiation, except for detecting the neutron. The discovery of the neutron revolutionized the way the atom was modeled. -
Werner Heisenberg
In 1932, Werner Heisenberg was awarded the Nobel Prize for his contribution to quantum mechanics. He also created the Uncertainty Principle, and helped us understand the behaviors of sub atomic particles. He said that electrons don't move in neat orbits like Niels Bohr thought, and he also studied the pattern of the electrons of atoms.
