Atomic Theory

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Atomic Theory


1704 Isaac Newton, an English physicist, proposed the idea that atoms are not stationary particles. He also suggested that a lightweight substance called ether propelled atoms to repel each other. Newton believed that atomic particles were attracted to each other (also known as forces). He stated that atoms were solid and unbreakable, and had different sizes, shapes, and properties. Just like how Democritus was opposed by other Greek philosophers, Newton's work was rejected by the church.

1900 Max Planck, a German physicist, created the quantum theory. While observing glowing, hot matter, he concluded that energy was being stored in packets. He called these packets quanta. He created an equation, known as Planck's constant, to find the amount of energy for each quantum. His formula was later used by other scientists such as Einstein, who was able to discover photons using the quantum theory.

1904 Hantaro Nagaoka, a Japanese physicist, created the first planetary atomic model. He believed that atoms were huge, positively charged spheres surrounded by rings holding lightweight electrons. It is also known as the Saturian model due to its rings. He believed that electrons moved in these rings around the nucleus. He proposed that the nucleus was positively charged. His model later influenced scientist Niels Bohr, who made a simmilar model.

1914 Henry Moseley, an English physicist, discovered that an element contains a specific amount of positively charged particles in its nucleus. He determined this using x-ray tubes. He named these particles "protons." This information was used to recreate the Periodic Table, since it had been arranged based on atomic mass. His discovery lead to the modern table with elements in order based on atomic number, or the amount of protrons it possessed in its nucleus.

1912 Earnest Rutherford, a scientist from New Zealand, determined that an atom is mostly empty space, but with a dense, positive nucleus. He determined this when he directed alpha particles at gold foil in a scattering experiment. He proposed that electrons were found outside of the nucleus, where they moved around randomly. Rutherford's model of an atom showed a positively charged nucleus with a negatively charged outer region.


1803 John Dalton, an English scientist, announced the Law of Multiple Proportions, or Dalton's Law. His law stated that when two or more elements combine with one another, the weight of each element are in a relationship. Dalton's Law soon led to the proposal of Dalton's Atomic Theory, which declared that matter is made up of small, indestructible, solid particles called atoms. It also stated elements only contain one specific type of atom, and that atoms can rearrange in a chemical reaction. He developed the concept of measuring atoms using moles. Dalton created his own model of an atom as a hard, tiny sphere without any internal composition. Dalton is also known today as the father of atomic theory for his contributions and discoveries.

1923 Louis de Brogile, a French physicist, introduced his theory of particle wave duality. He proposed that electrons have wave-like characteristics. He wrote a thesis for his doctorate degree stating that those particles may have wave qualities similar to light. De Brogile recieved a Nobel Prize for his theory in 1929. His ideas inspired the concept of wave mechanics.

1794 French chemist Joseph Proust opened up a whole new set of ideas when he created the Law of Definite Proportions (or Law of Constant Composition). This law declared that any two samples of a substance would have the same mass ratio for each element it contained. Proust discovered this by conducting an experiment with two samples, copper carbonate and artificial copper. He found that when he compared both substances, they had the same proportion of mass for each element (Oxygen, Copper, and Carbon). His law created a foundation for the Law of Conservation of Mass and the Atomic Theory. Proust's ideas were disapproved of by another French chemist by the name of Claude Berthollet, who believed that elements didn't always combine in constant proportions.


1919 Francis Aston, a British chemist, discovered isotopes. Aston invented the mass spectrograph, which he used in his experiments. The apparatus worked by focusing electromagnetic energy to seperate isotopes based on mass. He declared a rule of whoel numbers that stated that isotopes cannot have a decimal. His discoveries were crucial towards making developments in nuclear energy.

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1913 Niels Bohr, a Danish physicist, redeveloped another atomic model. He proposed that electrons were located in rings outside of the nucleus. He called these rings orbital shells. He acknowledged that electrons could only have a certain amount of energy depending on where it was located. However, it is now knownthis model only works for the element hydrogen.

1898 Marie Curie, a French chemist, discovered the elements polonium and radium. She also observed the elements uranium and thorium, from which she she developed the concept of radioactivity. She proposed that radioactive parts of an atom cause the atom itself to break down. She believed that an atom's ability to do such came from the interior of the atom. Her contributions helped lead to the development of the atomic bomb.

1897 J.J. Thomson, an English scientist, created the cathode-ray tube. He used this instrument to find an atom's polarity. He observed while experimenting that although an atom was mostly composed of electrons, there were some protons as well. Thomson created the plum-pudding model, which is a sphere made up of protons with electrons embedded in the shell. His work later helped Millikan determine the charge of an electron.

1886 Eugen Goldstein, a German physicist, discovered canal rays. He used a cathode-ray tube in his experiment. He knew that cathode rays, or electrons were positively charged. Since the canal rays traveled in the opposite direction in the tube, he concluded that they carried an opposite (positive) charge. This observation later led to the discovery of protons.

1909 Robert Millikan, a, found the amount of charge for an electron. He used an oil dropper mechanism to accomplish this. By using x-rays, he was able to ionize the air in the instrument, removing the electrons. The oil droplets seperated based on the number of electrons they contained. With this information, he was able to determine that the mass of an electron is 1000 times smaller than the lightest electron.

400 B.C. Democritus, a Greek philosopher, was the first to come up with the concept of atoms. He theorized that everything in the universe was made up of atoms and the space in which they existed. Democritus created the first atomic model as a small, hard sphere without any internal structure. He believed that atoms differed in mass, size, position, and arrangement, and that they were an indestructible mixture of solid, invisible, particles. Although he is considered today to be the father of modern science, Democritus' beliefs were rejected by other philosophers of the Greek era, including Aristotle.

1869 Dmitri Ivanovich Mendeleev, a Russian chemist, developed the first periodic table. He seperated sixty-three elements into seven distinct groups. He found that each element had a different mass, which determined its properties. His table made it easier to reference information about the elements. Also, Mendeleev's Periodic Table helped him predict new elements.

1874 G. J. Stoney, an Irish physicist , proposed that electricity is composed of electrons. He advocated that this subatomic particle was negatively charged. He dicovered the extent of an electron while performing an experimenting with the electrolysis of water. He used knowledge about the kinetic theory of gas

Atomic TimelineBy: Maylin Vititow

1926Physicist Erwin Schrodinger proposed that electrons have qualities similar to waves and particles. He created the atomic model of an electron cloud. He declared that it is not possible to find the exact location of an electron. However, he stated that orbitals could be used to find the electron's approximate location. He used mathematical approaches when creating this model of an atom.

1926 Werner Heisenberg, a German physicist, developed the Principle of Indeterminacy. This states that a particle's speed and position inside an atom is unknown. Heisenberg experimented with differnet velocities and postitions using methods called matrices. He created a model of an atom's nucleus, which contained protons and neutrons. He also discovered the allotropic forms of hydrogen, for which he was awarded a Nobel Prize.

1932 James Chadwick, an English physicist, discovered the neutron. He used wax and gamma rays in his experiments. Chadwick used radioactive polonium on the wax. He observed that protons had been released based on the particle's behavior. He found that neutrons had a relatively close mass to the mass of protons.

1951 Glen Seaborg, an American chemist, discovered ten transuranium elements. He and his team also created the first atom bomb. He discovered isotopes that are used in treating patients with radioisotope scans. Seaborg also proposed to add another row of elements onto the Periodic Table called the actinides. This suggestion led to a crucial development in redesigning the table as well as chemistry.


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