-
400 BCE
Democritus (460-370 BCE)
Democritus, an ancient Greek philosopher, theorized that everything was made up of small particles, setting the basis of the Atomic Theory. He believed that they were indivisible and indestructible. Because of this, he named those tiny particles, “atomos”, meaning indivisible. As an example he used a seashell to show how you can only cut it to a certain extent. -
Law of Conservation of Mass
Antoine Lavoisier stated the Law of Conservation of Mass, which stated that matter was neither created nor destroyed. This contributed to chemistry by giving an explanation that no elements are lost during a chemical reaction, for they only are rearranged. Additionally, this would further be used to support Dalton’s theories and it is still used today. -
Law of Multiple Proportions
John Dalton created the Law of Multiple Proportions, which states that whenever two elements combine to form more than one compound, there will only be a whole number ratio between the elements, even though they are in different compounds. This contributes to the atomic theory because Dalton then uses this in the future to back up his atomic theory, proving that you can’t have a fraction of an atom. -
Law of Definite Proportions
Joseph Proust’s Law of Definite Proportions, which followed the Law of Conservation of Mass, states that any compound always has the same elements in the exact same proportions by mass. This is significant because it led to the basis for chemical formulas and compounds. It also led to Dalton’s atomic theory, which happened later on. -
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 finally atoms can’t be created or destroyed. This theory is significant because it laid the foundation for chemistry itself, even if certain parts of it were proven wrong. -
John Dalton (1766-1844)
John Dalton, an English chemist, based his atomic theory on the Law of Conservation of Mass and the Law of Constant Composition. While certain parts of his theory may have been wrong, he reintroduced Atomic Theory into chemistry. He also made the Law of Multiple Proportions. -
Dmitri Mendeleev (1834-1907)
Dmitri Mendeleev, a Russian chemist, organized the periodic table, in which he arranged the elements by their atomic weight. He also arranged the elements in an easy-to-follow format which has been edited and added to this day. -
Henri Becquerel (1852-1908)
After X-rays were recently discovered, Henri Becquerel, a French physicist and engineer, investigated phosphorescent materials and found that Uranium and other substances also emit penetrating radiation. Through his investigations he discovered a phenomenon: radioactivity. Additionally, this showed that X-rays weren't the only type of penetrating radiation. -
J.J. Thomson (1856-1940)
J.J. Thomson, a British physicist, found the charge of an electron using cathode ray tubes and magnets. He determined that electrons were negatively charged and much smaller than the entire atom. He then went on to propose a model of the atom, which he called the “Plum Pudding Model”. -
Cathode Ray Tube Experiment
J.J. Thompson performed the Cathode Ray Tube Experiment to find that the electron is negatively charged and that the electron is much lighter than the atom.This is significant because he found that smaller particles made up atoms, disproving part of Dalton’s theory, and that this discovery helps us understand not only the structure of atoms, but also contributes to our understanding of chemical bonding. -
Marie Curie (1867-1934)
Marie Curie discovered Radium and Polonium, as well as developed the X-ray machine. With her observing Radium, she discovered that radiation wasn’t dependent on the molecular structure, but the atom itself. -
Plum Pudding Model
After J.J. Thomson’s cathode ray tube experiment and the discovery of the proton, many scientists were wondering what the structure of the atom was. Thomson proposed his “Plum Pudding Model” in which it is a positively charged sphere with electrons embedded into it. This is significant because it was the first model to introduce the subatomic particles, which introduced that the atom was divisible. -
Robert Millikan (1868-1953)
Robert Millikan, an American physicist, performed the Oil Drop Experiment, which 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, microscope, light source, oil, and two metal plates. The top plate was positively charged and the bottom was negative. They ionized the oil mist so that it'd be negatively charged and balanced the electrostatic and gravitational force. This let them measure the electrostatic and gravitational force, as well as the droplet's mass. 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 Thompson’s model of the atom, highlighting the fact that the protons and electrons were separate. It also clearly showed how the nucleus was packed with protons, with the electrons revolving around the nucleus. His model laid the foundation of the current model due to his correct depiction of the empty space. -
Ernest Rutherford (1871-1937)
Ernest Rutherford, a New Zealand physicist, found that the atom is mostly empty space, with all its mass centered in the nucleus from his Gold Foil Experiment. He stated that the nucleus was positively charged and the electrons were circling the nucleus in orbitals. -
Gold Foil Experiment
Rutherford and two other scientists performed this experiment by shooting alpha particles at a thin piece of gold foil, encased by a phosphorescent screen. Mostly, their experiment matched their prediction that the particles would shoot through easily, but some bounced back. From this they stated that an atom is mostly empty space and the middle is densely packed and positively charged. This is important because this is a key part of the atomic structure, contributing to the modern model. -
Niels Bohr (1855-1962)
Niels Bohr, a Danish physicist, made his own model of an atom, in which he specifically used Hydrogen. Even though this model is very similar to Rutherford’s model, he showed and stated that the electrons’ energy levels are discrete and that they can jump orbitals, given they absorb enough energy to do so. -
Bohr's Planetary Model
Niels Bohr, a Danish physicist, made his own model of an atom, in which he specifically used Hydrogen. Even though this model is very similar to Rutherford’s model, he showed and stated the the electrons’ energies are discrete and that they can jump orbitals, given they absorb enough energy to do so. -
Quantum Mechanical Model
After a scientist previously stated that all particles could be treated as matter waves, Erwin Schrodinger used his equation to create a new model of an atom. The model illustrated the three dimensional positions of electrons. This is significant because it accurately represents how electrons move in an atom (in waves) as well as it's three-dimensional position, increasing our overall understanding of the electron and chemical bonds. -
Erwin Schrodinger (1887-1961)
Using Bohr’s theory of an atom (electrons move orbitals they absorb and emit radiation), Erwin Schrodinger, an Austrian-Irish physicist, wanted to fit Bohr’s theory for not only Hydrogen, but for the rest of the elements. He created a wave equation that correctly found the energies of electrons in atoms. -
James Chadwick (1891-1974)
James Chadwick, an English physicist, performed an experiment with Beryllium and alpha particles and found an unknown radiation that was not either a proton or an electron, which was later discovered that it had no charge, making it neutral. He called this subatomic particle a neutron. He then made his own model, called the Chadwick’s Atomic Model, illustrating that protons and neutrons are in the nucleus, and the electrons in their orbitals. -
Murray Gell-Man (1929-2019)
Murray Gell-Mann, an American physicist, proposed that an even smaller particle, quarks, exist. He stated that particles like protons, electrons, and neutrons were made of quarks. He then proposed that quarks had a fraction of a charge and that they were held together by a different particle: gluons. At first, people didn’t believe him, but then after further experimentation he was proved to be correct.
