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Period: 1000 BCE to
History of the Atom
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400 BCE
Democritus
Democritus proposed that all matter is composed of indivisible and indestructible atoms. These atoms are always in motion and separated by a void. There are an infinite number of atoms with different shapes and sizes that determine the properties of the element. Incorrectly, the theory proposed atoms interacted with each other through various mechanical means such as hooks and sockets. The theory made many wrong assumptions about atoms, but was very similar to the modern model for its time. -
400 BCE
Aristotle
Aristotle believed that all matter contained a combination of air, earth, water, and fire. Aristotle also thought that matter could always be reduced to less matter until it became infinitesimally small. His assumption was based completely on natural philosophy and lacked scientific evidence to support it. It was also in direct contradiction with the modern atomic models and Democritus's theory. -
Antoine Lavoisier
Lavoisier discovered that mass is not created nor destroyed in chemical reactions. The mass of a substance created in a chemical reaction is always the same as the sum of the original reactants. For example, he understood atoms in an O2 molecule combine with atoms in an H2 molecule to create water, but the number of hydrogen and oxygen atoms remained the same throughout the reaction. He also pioneered stoichiometry by calculating the percent composition of water by element. -
John Dalton
Dalton applied the laws of conservation of mass and constant composition to predict that no mass was destroyed in chemical reactions. He also proposed that all matter is made of atoms, particles that are indivisible and identical in the same element. Furthermore, he hypothesized that compounds are combinations of different atoms. A chemical reaction is a rearrangement of atoms. Although his model didn’t address isotopes, subatomic particles, or nuclear reactions it was still very accurate. -
Henri Becquerel
Becquerel exposed phosphorescent uranium salts to the sun to attempt to prove his hypothesis that visible light can cause materials to emit x-rays. The hypothesis was false, but he stored the crystals on a photographic plate in the dark and proved they emitted radiation without light. While his initial theory was false, the discovery about radioactive decay wasn't. He didn't create an atomic model. -
J J Thomson
J J Thomson deflected cathode rays with an electric field, proving they’re composed of particles. He determined that the charge to mass of the particles were the same no matter what materials were used, proving they’re a universal part of matter. These electrons that he discovered were incorporated in his model of the atom. He proposed atoms are positively charged spheres embedded with negatively charged electrons. This is a rudimentary version of later models. -
Marie and Pierre Curie
Based on Henri Becquerel’s discover, Marie measured that the radiation emitted by uranium was proportional to its mass. She also knew that it wasn’t affected by external conditions and lasted for long periods of time. While measuring thorium radioactive decay, she noted that chalcolite and uranium pitchblende emitted more radiation than anticipated. As a result, she knew that they contained other radioactive elements and isolated polonium and radium. -
Max Planck
Planck proposed that walls of glowing solids contain resonators that oscillate at different frequencies, gaining thermal energy from the walls and losing it as electromagnetic radiation. He suggested the energy of the resonators was proportional to their oscillation frequency. In order to correlate with the spectrum, oscillator energy was limited to a specific range. Similarly, he demonstrated light was quantized because its energy was always a multiple of what is now the Planck constant. -
Robert Millikan
Robert Millikan determined the mass of an electron by spraying water droplets between 2 plates generating an electric field. X-rays ionized the air and droplets that captured the electrons were attracted to the positive plate at the top. Those that didn’t capture an electron fell to the bottom. By studying the drops, he determined the charge of the electron. Using this in the charge to mass ratio proved the mass of the electron, confirming J. J. Thomson’s prediction. -
Ernest Rutherford
Rutherford bombarded a 0.4 µm thick gold foil with –particles and Hans Geiger measured the scattering to be approximately one degree. Rutherford eventually discovered that very rarely, a –particle was scattered at an angle more than 90 degrees, suggesting it hit a highly concentrated and positively charged mass that repelled it. He then created a series of equations that predicted these deflections and created the modern atomic model consisting of a very dense, positively charged nucleus. -
Niels Bohr
Bohr proposed the modern atomic theory that a positively charged nucleus is surrounded by electrons that orbit it. He also proposed that the valence electrons of an atom affect its reactivity. These electrons could move from higher to lower energy orbits and emit a photon in the process. Finally, he developed the "liquid drop" model of the nucleus that was useful in understanding fission. Bohr was taught by J J Thompson and Ernest Rutherford. -
Henry Moseley
Contrary to the beliefs of Mendeleev, Meyers, and other scientists, Moseley demonstrated that properties of an element are determined by atomic number instead of atomic weight. He also proved that the charge of the nucleus depends on the atomic number. One of his most important accomplishments was Moseley’s law, a law that proved the frequency of X-ray radiation emitted by elements depends on its atomic number. -
Erwin Schrodinger
Schrodinger improved Bohr’s model of the atom by creating an equation that took into account electrons’ ability to behave as a wave or particle and didn’t define its exact path as in Bohr’s model. The equation calculated the energy of electrons in an atom and predicted the probability of finding an electron in a position, considering that it’s more likely to be in a dense part of the cloud. This quantum model was more applicable with complex atoms. -
Werner Heisenberg
Heisenberg proposed the uncertainty principle and that it isn’t possible to follow an electron orbit or find its precise position. These values should be represented by matrices because momentum and position contain errors greater than a specific constant. When momentum is measured precisely, position is less precise and vice versa. If the position of something is measured, it can’t always be predicted in the future because it lacked a definite position and momentum before it was measured. -
James Chadwick
In 1932, based on Frederic and Irene Joliot-Curie's experiments, Chadwick hypothesized that a theoretical neutral particle could split the nucleus. As a result, he used polonium to bombarded wax with protons. Protons were ejected from the nucleus and reacted with the same energy as if they had been hit by a neutron, proving that the particle existed. Contrary to the modern model, he initially anticipated that it consisted of an electron bound to a proton, cancelling the charge.
