-
200
Alchemists (200 B.C.)
Alchemists originated in the Near and Far East while combining Aristotle's ideas about matter, they tried to transmute ordinary metals into precious metals like gold, and also to create an elixir of life. Though they failed to reach these goals, Alchemists discovered many current chemical/medical properties, invented important pieces of chemical apparatus (vessels and distillation flasks), and studied/advanced many chemical processes (calcination, distillation, fermentation, and sublimation). -
332
Aristotle (332 B.C.)
Aristotle was a Greek Philosopher who proposed an atomic theory that was believed for almost 2,000 years and stated that all matter consisted of four elements: earth, air, water, and fire. He also believed that all matter consisted of four qualitlies: dryness, hotness, coldness, and wetness in that fire was dry and hot and water was cold and wet. -
450
Democritus of Adbera (450 B.C.)
Democritus was a Greek Philosopher who proposed the first atomic theory around 450 B.C., which stated that all matter is composed of "atomos", and that atoms remain unchanged when they combine with other atoms to form objects. He stated that atoms of the same element are the same, atoms of different elements are different, and lastly that the characteristics of a substance are determined by the shape of its atoms (sweet things=smooth atoms and bitter things=sharp atoms). -
Period: 450 to
Years of Contributions to the Atomic Theory (450 B.C. - 1951 A.D.)
-
Isaac Newton (1704 A. D.)
By studying gases, Newton beieved that we lived in a mechanical universe where small, solid masses are in motion and that atoms are held together with attractions (forces). He has been quoted as saying, "Matter is composed of hard and solid particles." -
Benjamin Franklin (1747 A.D.)
Benjamin Franklin studied static electricity, which can be defined as electrical charges that are not in motion. He came to some important conclusions after his famous kite experiment such as: objects can either have a positive or a negative charge, particles with like charges repel, particles with opposite charges attract, and excess negative charges build up on objects. -
Antoine Lavoisier (1789 A.D.)
Antoine Lavoisier was a French chemist who discovered the Law of Conservation of Mass, which states that although matter can change form through a chemical reaction, it's mass will stay the same throughout the reaction. This was an extremely important discovery, as it is still regarded today as true and is included as a part of the modern atomic theory. -
Joseph Proust (1799 A.D.)
Joseph Proust discovered the Law of Constant Composition in 1799 which stated that a given compound always contains the same elements in the same proportions; he also believed matter could be combined in specific patterns to make bigger, more unique matter. Proust's beliefs and findings played a big role even further because they were the basis and evidence that allowed John Dalton to propose the Law of Multiple Proportions and create his own atomic theory. -
John Dalton (1803 A.D.)
John Dalton was an English chemist who in 1803 proposed an atomic theory that stated all matter is composed of small particles called atoms and that all atoms of the same element are identical, while atoms of different elements have a different size and mass. He also proposed that chemical compounds are composed of atoms in defined ratios, chemical reactions result in a rearrangement of atoms, and atoms cannot be subdivided or created/destroyed. -
Michael Faraday (1832 A. D.)
Michael Faraday was an English chemist who came up with the idea of "electrolysis" (the separation of molecules with electricity). Through diligent research on this subject, Faraday suggested that the structure of atoms was somehow related to electricity. -
Dmitri Mendeleev (1869 A. D.)
Dmitri Mendeleev created the first periodic table in 1869 by arranging elements that had similar properties into seven groups. He also published the Periodic Law that stated the properties of elements are periodic functions of their atomic numbers. -
William Crookes (1879 A. D.)
William Crookes did extensive experimentation with cathode ray tubes. These experiments led him to the assumption that cathode rays move in straight lines, cause glass to glow brightly, bestow a negative charge on objects they come into contact with, have a negative charge because they repel magnets, and lastly they have mass because they led to the movement of pinwheels that were in their pathway. -
Eugen Goldstein (1886 A. D.)
Eugen Goldstein used a cathode ray tube to discover canal rays, whose electrical and magnetic properties are the opposite of that of an electron. To some, he is considered the man who discovered protons, and he also studied discharge tubes to help pinpoint the influence magnetic fields had on their glow. -
G. J. Stoney (1894 A.D.)
G. J. Stoney worked to determine the magnitude of an atom or particle of electricity (what he called an electron) and stated that the atom or particle of electricity be one third of the fundamental units in a system of physical units that should be created. Stoney ended up calculating the magnitude of an electron from data obtained in an electrolysis experiment, and theorized that electricity was comprised of negatively charged electrons. -
Joseph John Thomson (1896 A. D)
Joseph John Thomson conducted a cathode ray experiment with cathodes (negatively charged electrode he then called electrons) and anodes (positively charged electrode) in 1896. From this experiment, Thomson hypothesized that atoms need particles with a positive charge to balance out the negative charge of electrons to make atoms neutral, and also atoms need to have other particles that account for most of their mass since electrons are low in mass. -
Henry Becquerel (1896 A.D.)
Henry Becquerel stuided the effects of X-Rays on photographic film and conducted an experiment where he placed a piece of uranium on an unexposed photographic film. Through this experiment, he discovered radioactivity (the spontaneous emission of radiation from an element) and showed that some chemicals can release very penetrating rays when they decompose. -
Joseph John Thomson (1897 A.D.)
In 1897 Joseph John Thomson used the results of his Cathode Ray Experiment to determine the charge to mass of electrons (1.759 x 10^11 coulombs/gram) and also by studying "canal rays" he found that they were associated with the H+ proton. With all of his knowledge and data from his experiments, Thomson assumed all the negatively charged particles were in a sphere of positive charge as depicted in "Thomson's Plum Pudding Model". -
Marie and Pierre Curie (1898 A.D.)
Marie and Pierre Curie examined thorium and uranium and named their decay process "radioactivity", which helped provide further context into understanding the characteristics of radioactive atoms. Furthermore, they also isolated the radioactive elements known as polonium and radium. -
Ernest Rutherford (1898 A.D.)
Rutherford conducted a radiation experiment in which he examined radiations emitted from uranium and thorium. In total, he noticed a high deflection of negatively charged and low mass radiation and called this beta radiation, no deflection of neutral radiation calling this gamma radiation, and a small deflection of high mass (2+ charge) radiation calling this alpha radiation. -
Max Planck (1900 A.D.)
Max Planck was a German scientific philosopher who is known by many as the founder of the quantum theory because he utilized discrete units of energy and/or quanta to justify how hot, illuminated objects emit ight. This helped to give us a better understanding of the movement of electrons, and showed us that energy can be determined in frequency and not intensity because energy doesn't flow continuously (energy delivered in discrete packets). -
Hantaro Nagaoka (1903 A.D.)
In 1903 (10 years before Bohr's famous model), Hantaro Nagaoka proposed a quasi-planetary model of the atom that consisted of flat rings of electrons orbiting around a positively charged particle. Though Nagaoka's model had some problems such as lack of stability for electron orbitals, it was an important atomic model and provided the basis for Bohr and many future scientists to base their model off, while making small improvements and adjustments to it here and there. -
Richard Abegg (1904 A.D.)
Richard Abegg discovered that inert gases have a stable electron configuration; this gave an explanation for their chemical inactivity. He also proposed the Valence Bond Theory that explained how atoms bond with one another and led to the further understanding of the principles of ionic bonding. -
Albert Einstein (1905 A.D.)
Albert Einstein presented the famous equation E=mc^2, worked extensively in the field of theoretical physics, and published a number of scientific papers (example: The Special Theory of Relativity). Albert published papers concerning the nature of light that became a cornerstone of the quantum theory such as the Photoelectric Effect (many metals emit electrons when light is shined upon them) and lastly, he put forth more analysis into the aspects of molecular motion and the particle theory. -
Robert Millikan (1909 A.D.)
Robert Millikan conducted an Oil Drop experiment in 1909 that determined the charge (charge = 1.60 x 10 ^-19 coulomb) and mass (mass = 9.11 x 10^-28 grams) of an electron. -
Ernest Rutherford (1911 A.D.)
Rutherford conducted an experiment where he shot alpha particles through a piece of gold foil and noticed that while most of the alpha particles went straight through the foil, some were deflected back. He assumed the atom is spherical and has a positively charged nucleus in the center with diffusing negative charge surrounding it. His conclusions became the bases of his planetary model (positively charged nucleus with negatively charged electrons surrounding and orbiting it). -
Henry Moseley (1914 A.D.)
Through X-Ray Frequency experiments, Henry was able to determine the charges on the nuclei of most atoms and concluded that how many protons an element has is equal to the element's atomic number. This was a tremendous discovery because it showed us that an element is defined by its number of protons, and also when Moseley arranged the elements in the periodic table based on their atomic numbers, some of the problems and inconsistencies that were plaguing the periodic table went away. -
Francis William Aston (1919 A.D.)
While using a mass spectrograph, Francis William Aston discovered the existence of isotopes (identified 212 isotopes). This was a major discovery for the atomic theory because it showed us that it is possible for atoms of the same element ot have a different number of neutrons. -
Niels Bohr (1922 A.D.)
Niels Bohr worked with Ernest Rutherford and used Rutherford's description of the nucleus to discover what occurs inside of an atom and to develop a picture of the atomic structure. Niels discovered that electrons move in separate orbitals surrounding the nucleus, and he concluded that the number of electrons in those orbitals will determine the properties of an element. -
Louis de Broglie (1923 A.D.)
Louis greatly contributed to the atomic theory because he thought that electrons acted as particles and waves, and also that waves produced from electrons in orbitals around the nucleus create a standing wave of wavelength, frequency, and energy. This helped scientists better understand electron charateristics and proved that electrons don't move in completely regular orbits. -
Werner Heisenberg (1927 A.D.)
Werner Heisenberg had a couple of important discoveries including: The Uncertainty Principle in which he stated that electrons don't travel in orbits and by developing "matrix mechanics" he was able to specifically describe quantum jumps and the physical properties of particles more accurately. The Uncertainty Principle also concluded that it is impossible to know both the position and velocity of a particle with exact precision. -
Paul Dirac (1930 A.D.)
In 1930, Paul Dirac helped change the world of atomic theory because while originating an equation for electron behavior, he proposed the existence of antiparticles. He stated that every particle in existence corresponds to an antiparticle that exactly matches the particle, but with an opposite chage; he concluded that discharges of energy can be produced from these particles making contact. -
Erwin Schrödinger (1930 A.D.)
Erwin was an Austrian physicist who utilized mathematical equations and "wave mechanics" to predict the likelihood of an electron being in a certain position in an atom. His atomic model known as the "Quantum Mechanical Model of the Atom" introduced the idea of sub-energy levels and depicts a nucleus being surrounded by an electron cloud, and where the cloud is the most dense, it's most likely there will be an electron there and vice versa for areas of low density. -
James Chadwick (1932 A.D.)
On January 1st 1932, James Chadwick influxed beryllium atoms with alpha particles and noticed that a radiation with a neutral charge was produced that had a similar mass compared to a proton; James concluded that this particle be called a neutron. Overall, Chadwick discovered the neutron, which allowed for a better model of the atom to be available to chemists, and also concluded that the atomic number can be determined by counting the number of protons in an atom. -
Enrico Fermi (1942 A.D.)
Fermi further evolved the Beta decay theory and demonstrated that nuclear transformation happens in the majority of elements hit with neutrons; these conclusions led to the discovery of new elements, slow neutrons, and nuclear fission. Fermi conducted the first ever controlled chain reaction of releasing energy through an atom's nucleus and also played a big role in solving problems for the first atomic bomb, which showed first hand how far our knowledge of the atom and atomic theory had come. -
Glenn Seaborg (1951 A. D.)
Glenn Seaborg discovered and synthesized six transuranium elements, helped to discover plutonium, and also discovered more than 100 isotopes on the periodic table. His discoveries and research helped the atomic theory by providing more elements to the periodic table, and discovering a significant amount of isotopes that gave specific details to the chemical makeup of atoms of the same element.
