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400 BCE
Democritus
Democritus was an ancient Greek philosopher who theorized that everything is made up of indivisible, indestructible, small particles. He named these particles, "atomos", which means "uncuttable" in Greek. His first atomic model was a round sphere without protons, neutrons, and electrons. His contribution set the basis of the Atomic Theory and allowed scientists to further look into the science of the atom. -
Law of Conservation of Mass
The Law of Conservation of Mass, which was discovered by Antoine Lavoisier, stated that matter was neither created nor destroyed. This helped further chemistry by clarifying that substances did not disappear during chemical reactions, but were rearranged. Later on, this would be used to support Dalton's theories and the law is still used today. -
Law of Definite Proportions
The Law of Definite Proportions, which was discovered by Joseph Proust, states that a compound is always composed of the same elements in the exact same proportions by mass. This is significant because chemical formulas and compounds, today, were heavily based on this law. -
Law of Multiple Proportions
The Law of Multiple Proportions, created by John Dalton, states that whenever two elements combine to form more than one compound, there will only be a whole number ration between the elements, even though they are in different compounds. This is important because it supports a part of Dalton's theory which he would come up with in 1808 that states atoms as indestructible. -
John Dalton
John Dalton was an English chemist, physicist, and meteorologist. He is best known for introducing his atomic theory to the world of chemistry. It is based on the Law of Conservation of Mass and the Law of Constant Composition. Even though we have now proven some of his theory wrong, most of his theory is still accepted today. He has also made several laws before, such as the Law of Multiple Proportions. -
Dalton's Atomic Theory
John Dalton's Atomic Theory stated that all matter is composed of indivisible and indestructible atoms, all atoms of the same element are identical in mass and properties, compounds are composed of two or more different atoms from different elements, a chemical reaction is a rearrangement of atoms, and atoms can't be created or destroyed. This theory is significant because it helped lay the foundation of chemistry, even if there are certain parts that are now proven incorrect. -
Dmitri Mendeleev
Dmitri Mendeleev organized the periodic table which consisted of the elements by their atomic weight. His arrangement of elements with a few additions is used quite frequently among scientists, even today. -
Henri Becquerel
Henri Becquerel, a French engineer, investigated phosphorescent materials and found that Uranium and other substances emit penetrating radiation after the discovery of the X-ray. Through his investigations, he discovered a phenomenon called radioactivity. This finding showed that X-rays weren't the only type of penetrating radiation. -
J.J. Thomson
J.J. Thomson was a British physicist. He experimented with cathode ray tubes and magnets to determine that electricity is negatively charged and that it was much smaller than atoms. He also proposed a model of the atom which he called the "Plum Pudding Model." -
Cathode Ray Tube Experiment
Using a cathode ray tube and an anode and cathode on either side, J.J. Thomson determined that the ray was negatively charged and determined the mass to charge ratio and found that these particles were lighter than atoms. This is important because he found that smaller particles made up atoms which disproved Dalton's theory. This helped us understand the structure of atoms and contributed to our understanding of chemical bonding. -
Marie Curie
Marie Curie discovered Radium and Polonium and also developed the X-ray machine. While observing Radium, she discovered that radiation wasn't dependent on the organization of atoms, but rather the atom itself. -
Plum Pudding Model
After J.J. Thomson's cathode ray tube experiment, the proton was also discovered. This discovery led to many scientists wondering what the structure of the atom was and how it looked like. In Thomson's Plum Pudding Model, there is a positively charged sphere in which electrons are embedded into it. These electrons were thought to be moving. This model is significant because it was the first model to introduce the subatomic particles which showed that the atom was, in fact, divisible. -
Robert Millikan
Robert Millikan performed the Oil Drop Experiment. This experiment determined the magnitude of an electron's charge. He also determined that the electron's fundamental charges were simple multiples of a simple integer. -
Oil Drop Experiment
Millikan and Harvey Fletcher used an atomizer, a microscope, light source, and two metal plates. They first ionized the oil mist, making the particles have a negative charge. The top plate was positively charged and the bottom was negatively charged, they then balanced the electrostatic force and gravitational force, allowing them to measure the droplet's mass, gravitational force, and electrostatic force. This is important because they proved the existence of the electron and its exact charge. -
Rutherford's Model
After his gold foil experiment, Rutherford revised Thomson's model of the atom. He clearly illustrated that the protons and electrons were separate and he showed how the nucleus was packed with protons and neutrons, which electrons revolved around the nucleus. His model laid the foundation of the current model due to the accurate representation of the nucleus and the empty space. -
Gold Foil Experiment
Rutherford and his team performed this experiment by shooting alpha particles at a thin sheet of gold foil, which was encased by a phosphorescent. For the most part, their experiment matched their predictions which was that the particles would go through, though some did bounce back. Using this, they stated that an atom is mostly made of empty space and the middle is densely packed and positively charged. This is important because it is a key part of the atomic structure, even today. -
Ernest Rutherford
Ernest Rutherford, a New Zealand physicist, performed the Gold Foil Experiment and found that the atom is mostly empty space in which all its mass is centered into the nucleus. He stated that the nucleus was positively charged and the electrons were circling the nucleus in orbitals. -
Bohr's Planetary Model
Bohr's model showcased the atom similar to our solar system in which electrons would orbit the nucleus in distinct shells like our planets circle the Sun. He also stated that each of the orbitals is an energy level and that electrons can jump orbitals if they are given the right amount of energy. This is important because it explains why atoms would only emit certain light at certain wavelengths. This same principle would later be used on theories for photons. -
Niels Bohr
Niels Bohr was a Danish physicist, who made his own model of an atom, specifically Hydrogen. While it was very similar to Rutherford's model, the difference was that he stated that the electrons' energies are discrete and that they can jump orbitals if they absorb enough energy. -
Erwin Schrodinger
Using Bohr's theory of an atom, Erwin Schrodinger wanted to fit Bohr's theory for all the elements, not just Hydrogen. He created a wave equation that correctly found the energy levels of electrons in atoms. -
Quantum Mechanical Model
Erwin Schrodinger used his equation to create a new model of an atom after he found that all particles could be treated as matter waves. The model showed the 3D positions of electrons. This is significant because it properly represents how electrons move in an atom. This increased our overall understanding of the electron and its chemical bonds. -
James Chadwick
James Chadwick performed an experiment with Beryllium and alpha particles and found an unknown radiation that was not either a proton or an electron. He called this subatomic particle a neutron because it has no charge and he also made his own model, called the Chadwick's Atomic Model to include neutrons into the known structure of an atom of that time. -
Murray Gell-Mann
Murray Gell-Mann, an American physicist, proposed that an even smaller particle, quarks, existed. He stated that the subatomic particles (protons, neutrons, and electrons) were all made of quarks. He also said that quarks had a fraction of a charge. Most people didn't believe him at first, but after further experimentation he was proven to be correct.