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Period: 200 to
Development of Modern Atomic Theory
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300
Aristotle
Around 300BC Aristotle didn’t believe that atoms were of different sizes and had regular geometric shapes which contradicted societies belief for his day. He believed all matter contained fire, air, earth, and water and emotions of the world contributed to good and bad. -
430
Democritus
Around 430BC Democritus made very significant observation for his lifetime he stated that atoms are the building blocks for all things and that atoms are tiny, indivisible, and differ only by shape and arrangement. He also stated that atoms cannot be destroyed and that atoms correspond to the substance that they make. -
Antoine Lavoisier
In 1778 Lavoisier did many experiments with gases, he basically discovered oxygen and hydrogen and their names stuck. When some substances were burned he stated that their loss in mass was gas molecules escaping into the atmosphere which led him to establish the Law of Conservation of Mass. -
Joseph Louis Proust
In 1799 Proust was studying sugar composition in Spain which led him to the Law of Constant Composition (later merged into the Law of Definite Proportions) stating no matter how a compound is produced or how much there is of it the reactants would always combine in the same way. -
John Dalton
In1803 after many years of researching atoms John Dalton published his Theory on atoms which stated;
• All elements are made up of tiny indivisible particles, known as atoms
• Atoms of the same element are identical with respect to their weights
• Atoms of different elements are different from each other and can be identified by their relative weights
• Atoms can neither be divided into smaller particles nor destroyed
He also expanded upon the idea of definite proportions and the Law of Multiple -
Michael Faraday
Michael Faraday mostly studied electricity but he also studied gases such as chlorine and he would diffuse them. He also studied elements forming into compounds. In 1832 he studied the effects of electricity on atoms and coined the phrase electrolysis -
Marie & Pierre Curie
Marie & Pierre Curie discovered two new elements and did vast research on radioactivity in 1896. Which helped shape the understanding of radioactive atoms and their unique properties. -
Henri Becquerel
Henri was experimenting with an ore containing uranium in his lab and found on accident that if he left it on a photographic plate without any light the ore still left an image. That happened because uranium is radioactive and that was the first time it was documented as such. -
JJ Thomson
In 1897 scientist JJ Thompson created a tube with a positively charged anode side and a negatively charged cathode side which made sort of a beam then he placed a magnet in the middle and the beam bended toward the positive end of the magnet meaning the particles in the beam were negatively charged thus electrons. -
Max Planck
In Dec. of 1900 Planck discovered that electromagnetic energy is released in quantized specific amounts. In the formula to find the amount of energy that is released when electrons jump energy levels, h is plancks constant -
Ernest Rutherford
Ernest Rutherford did his famous gold foil experiment in 1908. He shot alpha particles at gold foil, most of them went right through the foil but some were reflected meaning there was a small, dense, positive charged area he called it a nucleus of an atom. -
Albert Einstein
In 1905 Albert Einstein used the idea of quanta (energy given off as light when an electron moves from a lower energy level to a higher one) to figure out the photoelectric effect in more detail. Which helped scientists learn where the electron clouds might be. -
Robert Millikan
In 1909 Robert Millikan with his oil-drop experiment figured out that the mass of an electron was very small and that the overall charge of an atom is neutral. -
Niels Bohr
Niels Bohr expanded upon his teacher Ernst Rutherford’s idea in 1913 Bohr stated that electrons stay in “clouds” and the farther away you get from the nucleus the more electrons these clouds can hold and the outer ones are what distinguishes the atoms chemical properties and when electrons jump form an outer orbit to an inner one light is emitted. -
Werner Heisenberg
Heisenberg created the uncertainty principle in 1925 that states the more precisely the position of a particle is determined, the less precisely the momentum is known in this instant, and vice versa. -
Wolfgang Pauli
Wolfgang Pauli is mostly known for his Pauli Exclusion Principle which states that no two electrons in an atom can have the same four quantum numbers. This allows scientist to give electrons an “address” to further distinguish themselves from others in the same atom. -
Friedrich Hund
Friedrich Hund established Hund’s principle which states that electrons must fill one electron in every sublevel before doubling up. This goes along perfectly with the Pauli Exclusion Principle. -
James Chadwick
In 1932 James Chadwick discovered a new subatomic particle that he named the neutron. They have no charge and their function is to stabilize the nucleus of an atom by allowing protons to be close to one another. He discovered them by observing when Beryllium was hit with alpha particles it emitted a strange light and the substance that was emitting the light had no charge.
