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370 BCE
Democritus Discovers Atoms
Democritus named the unknown particle an "atom", after the Greek word "indivisible". He used the basis of the elements (earth, fire, wind, and air) to prove the shape and size of the atoms. This discovery kicked off the atomic theory that we study today. -
Lavoisier Discovers Elements
In 1772, Lavoisier burned a diamond in a glass jar with a magnifying glass, and discovered that the gas produced was the same as charcoal, and that both substances were variations on carbon. In 1779, he coined the element oxygen when he discovered it was released from mercury oxide, and found that oxygen was necessary for combustion and respiration, and that it composes 20% of the atmosphere. In 1777, after extensive experiments, he identified sulfur as an element. -
John Dalton and his Model
Based on experiment results, Dalton proposed the modern theory of the atom. He stated that all matter is composed by indivisible atoms, all atoms of an element are the same, and the separating or rearranging of atoms cause chemical reactions, which cannot change an atom of one element into an atom of another. He also said that compounds are structured by atoms of different elements in simple, whole number ratios. -
J.J. Thomson Discovers Electron
In 1897, Thomson discovered the electron through his experiments with a cathode ray. He studied positively-charged particles in a neon atom with the ray (which was negatively charged), and found that there are negatively-charged particles in the atom. This lead to his proposal of the "plum-pudding" model, and his discovery changed the way that atoms were modeled. -
Plum Pudding Model
In 1897, Thomson proposed the "plum pudding" model of an atom to try to explain some of the electrical properties of the atom after he discovered the electron. He thought that the "dough" (atom) was positively charged, with the "raisins" (electrons), which were negatively charged, were scattered throughout. The fault with this model was that it failed to recognize that the positive charge in an atom was due to a positively-charged particle (proton) instead of the whole atom. -
Hans Geiger Detects Alpha Particles
Hans Geiger introduced the first successful detector of individual alpha particles in 1908 while working with Rutherford. They were able to use Geiger's device to identify the alpha particle as the nucleus of a helium atom, which lead to Rutherford's claim in 1912 that the nucleus occupies a very small volume in the center of the atom. -
Ernest Rutherford Discovers the Nucleus
In 1910, Rutherford discovered the nucleus through his experiments with the scattering of alpha rays and the inner structure of the atom. This is discovered with his work on detecting alpha particles with Geiger. He declared that practically the whole mass of an atom is concentrated in a minute space in the center, along with positive particles. -
Bohr and the Planetary Model
Using classical mechanics and electromagnetic theory, Bohr developed Rutherford's model into the planetary model by requiring that electrons move in fixed orbits of size and energy, and when electrons move from one ring to another, they release energy, which explained radiation. -
Rutherford and Atomic Numbers
In 1913, working with H.G. Mosely, Rutherford bombarded atoms of various elements using cathode rays, and showed that the inner structures correspond with a group of lines which characterize the elements. Each element was then assigned an atomic number, and the properties of that element could be defined with that number. -
Millikan Measures the Charge on an Atom
Between 1908 and 1917, Millikan measured the charge on an electron through his own apparatus. The atomizer on a perfume bottle sprayed water or oil droplets into a sample chamber, which could be observed with a microscope. X-rays were beamed through. Some droplets caught the electrons and floated to a positively-charged plate at the top. He found that the charge was always a multiple of 1.59x10 -19C. Combined with Thomson's hypothesis, it proved an electron was 1000 times smaller than the atom. -
DeBrogelie's Theory
In 1924, he created a revolutionary theory of electron waves, which expanded on the restricted motion of electrons. According to Britannica, “A wave confined within boundaries imposed by the nuclear charge would be restricted in shape and, thus, in motion, because any wave shape that did not fit within the atomic boundaries would interfere with itself and be canceled out.” -
Hund’s Rule of Maximum Multiplicity
In 1925, Hund came up woth his maximum multiplicity rule, which dealt with quantum physics. He said that “a greater total spin state makes the atom more stable.” -
Schrödinger and the Electron Cloud Model
In 1926, he developed the quantum mechanical model of the atom, which predicts the possibility of electrons in a certain position. In the model, the nucleus is surrounded by the electron cloud, and the odds of finding an electron are highest where the cloud is most dense. -
Hans Geiger and the Geiger-Müller Counter
In 1928, Hans Geiger introduced this device, which was able to detect both alpha and beta particles (electrons) and ionizing photons. This device marked the introduction of modern electrical devices into radiation research. -
Chadwick Discovers the Neutron
Chadwick studied the then-unidentified radiation from beryllium as it hit a paraffin wax target, and concluded that the radiation was a neutral particle about the size of a proton, which he named the neutron. By 1934, the neutron had become accepted as a fundamental particle (as opposed to a proton and an electron melded together). His discovery led to the development of the atomic bomb, and Chadwick himself worked on the Manhattan Project during WWII.
