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500 BCE
Alchemists (500 BC)
They applied Aristotle's methods of developing a substance that they thought would turn base metals into fake gold. -
442 BCE
Democritus (442 BC)
Democritus developed the idea that atoms are homogeneous and had no internal structure. He also thought that atoms differed in shape, size, position, arrangement, and mass. -
Period: 442 BCE to
History of the Atom
Here is the historical timeline of the atom, starting from 442 BC to 1932. -
Joseph Louis Proust (Late 1700's)
Proust came up with the idea that compounds have chemical formulas. He also performed experiments and observed how different elements reacted in proportions with oxygen. -
Law of Conservation of Mass
Antoine Lavoisier conducted many experiments with dephlogisticated air, which was later called oxygen, and made a theory that the gas made substances acidic. His theory was incorrect, but later realized that water was formed by the reaction of hydrogen and oxygen. From this, he established this law which states that mass is not lost or gained during chemical reactions. -
Antoine Lavoisier
He conducted experiments with dephlogisticated air, in which he later renamed it as oxygen. He also established the Law of Conservation of Mass, which states that mass is not lost or gained during a chemical reaction. -
Law of Definite Proportions
French chemist Joseph-Louis Proust proved this law by using oxides and iron, and noted that metals always combine with two proportions of oxygen. The law states that samples of a compound contain an equal proportion of elements by their mass. -
Dalton's Atomic Theory
This theory suggested that atoms were the smallest particles that made up matter, and all matter was based upon a single unit. This theory also suggests that atoms can't be created or destroyed, atoms of different elements combine to form compounds, and all atoms of elements are identical. -
John Dalton
Dalton used Proust's Law of Proportions to developed the first modern atomic theory and realized that all matter must be composed of tiny particles. He theorized that all matter is composed of atoms, all atoms are identical and that all atoms of different elements have different properties, and all chemical reactions are involved in the combination of atoms. -
Dmitri Mendeleev
He worked on the periodic law and created the first periodic table. The law states that when elements are arranged by their atomic number, elements with similar properties appear at regular intervals. -
Cathode Ray Tube (Late 19th Century)
J.J. Thomson placed two oppositely-charged electric plates around the ray and saw that the ray was composed of negatively-charged particles. He also observed the magnetic field of the particles by using magnets. From that, he concluded that these subatomic particles can be found within any atom. -
Plum Pudding Atomic Model (Late 19th Century)
J.J. Thomson developed this model to show that electrons in an atom were negatively-charged particles within a sea of positive charges. -
J.J. Thomson
He developed the Plum Pudding model, which showed that each atom is filled with a positively charged fluid and inside are small, negatively charged electrons. -
Gold Foil Experiment
Ernest Rutherford performed this experiment by firing positive alpha particles at gold foil and measuring the deflection as the particles came out. Not many particles would deflect, so he concluded that there had to be a positive center in the foil, which he called the nucleus. -
Ernest Rutherford (1908-1911)
He conducted experiments on the gold foil he designed by charging positively-charged particles at the gold. Some particles were deflected at small angles, and some flied straight back. From this, he suggested that atoms are made up of empty space and that there was a dense, positively-charged area in the atom that caused the repulsion of the particles. -
Niels Bohr
Bohr took the concept of the model of an atom and applied new ideas of the quantum theory to it by explaining new evidence that challenged the old model and formulating the new and improved model. -
Henry Moseley
By observing and measuring the x-ray spectra of ten elements, Moseley discovered that the energy of an x-ray depended upon the nuclear charge of an atom. He established the atomic number as an experimental quantity and gave it a physical meaning. He also thought that elements could be arranged by their atomic number than by their weight. -
Oil Drop Experiment
Robert Millikan conducted this experiment to see if he could calculate the specific value of the electron's negative charge. He did this by observing two droplets of oil falling between two electrical plates that were ionized by x-rays. From this, he found that there were differing charges on different oil drops, which was caused by the different numbers of electrons. -
Nuclear Model (1911-1914)
Ernest Rutherford developed this model over a series of experiments and showed an atom with a dense, positively-charged area of the atom, which is known as the nucleus. -
Planetary Model
Niels Bohr developed this model to show his theory that electrons orbit the nucleus with a set and energy, and that energy is related to the size if its orbit. He also concluded that when an electron transfers to another shell, radiation is either emitted or absorbed. -
Robert Millikan (1908 & 1917)
He confirmed Thomson's hypothesis of a charge to a mass ratio by measuring the charge on an electron with his oil-drop apparatus. From that, he discovered the value of an electron's charge. -
Werner Heisenburg (1920's)
He developed a new approach on coming up with a quantum mechanics that could be observed and conducted experiments by measuring the position of an electron with a gamma ray microscope. -
Erwin Schrodinger
He combined equations for wave behavior with the de Broglie equation to formulate a mathematical model for the distribution of electrons in an atom. His model shows the probability of the region of an electron. His model also describes it as a wave and alienates the region where electrons are most likely to be found. -
Quantum Mechanical Model
Erwin Schrodinger used mathematical equations to describe the likelihood of finding an electron in a certain position. The model does not define the exact path of the electron, but it predicts the odds of the electron's location. -
James Chadwick
He produced an unknown radiation by bombarding beryllium atoms with alpha particles. He interpreted this radiation as a neutral electric charge that contained the mass of a proton. This neutral electric charge became known as the neutron.
