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Timeline of Atomic Structure. Marissa Kopelman. Democritus (460 B.C.-370 B.C.). Democritus, a Greek philosopher, believed that all matter was made up of indivisible and indestructible particles called atoms.

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democritus 460 b c 370 b c
Democritus (460 B.C.-370 B.C.)
  • Democritus, a Greek philosopher, believed that all matter was made up of indivisible and indestructible particles called atoms.
  • His ideas did not explain chemical behavior and were not based on the scientific method, having no experimental support.
  • His theory was a response to that of Parmenides and Zeno, who believed that the universe was formed of material that was a mass that had no empty space.
  • Other Greek philosophers such as Aristotle and Plato challenged and dismissed Democritus’ ideas.
1803 john dalton
1803John Dalton
  • The modern age of atomic discoveries began with John Dalton, an English chemist and schoolteacher.
  • Dalton formed a scientific theory through experimental methods based on Democritus’ ideas.
  • He formed hypotheses based on his experiments and his study of the ratios in chemical reactions in which elements combine.
  • Dalton used Lavoisier’s Law of Definite Proportions, established in 1799, in his experiments combining elements and proposed the Law of Simple Multiple Proportions.
1803 john dalton continued
Dalton’s Atomic Theory was the result of his observations:

All elements are made of indivisible particles called atoms.

All atoms of the same element have identical properties and mass.

Compounds are formed by the combination of atoms of different elements.

Chemical reactions are the separation, combination, or rearrangement of atoms.

Dalton’s atomic symbols

1803John Dalton (continued)
1896 marie curie
1896 Marie Curie
  • In 1896, French chemist Antoine Henri Becquerel made an accidental discovery when he was studying uranium salts exposed to sunlight and their ability to fog a photographic film plate. He left one sample that was not being exposed to the sun on the plate and saw that it had fogged. His associates Marie Curie and her husband Pierre Curie showed that the uranium atoms emitted rays that fogged the plates.
  • Marie Curie, a Polish scientist, named the process that the uranium atoms gave off rays, radioactivity. The particles and rays given off by a radioactive substance was called radiation. This discovery proved that Dalton’s theory that atoms are indivisible was incorrect. Radioisotopes, or radioactive atoms, have unstable nuclei and emit radiation as they undergo changes.
  • Curie discovered that the radioactivity of a substance was in proportion to the amount of the radioactive material present.
  • The discovery of radiation allowed Becquerel and the Curies to be awarded the Nobel prize in 1903.
1897 j j thomson
1897 J.J. Thomson
  • Thomson, an English physicist, discovered the electron through experiments involving electric currents passing through low pressure gases.
  • Gases were sealed in glass tubes with electrodes, metal disks, at each end, which were connected to an electricity source. The anode, one electrode, became positively charged as the other, the cathode, became negatively charged. A cathode ray, a glowing beam, then traveled to the anode from the cathode.
  • Thomson used electrically charged plates and a magnet to observe how the cathode ray repelled the negative charge. He hypothesized that the cathode ray consisted of fast moving negatively charged particles which he called corpuscles, later called electrons.
1897 j j thomson continued
Thomson also performed another experiment to test his hypothesis in which he measured the electron charge’s ratio to its mass. This ratio was constant and was not affected by anything in the experiment, such as the type of gas. This led to his conclusion that electrons are a part of the atom of every element.

At the time of this discovery, Thomson knew that the same charges repelled each other and the opposite charges attracted.

Thomson’s results allowed him to develop the first model of the atom with Lord Kelvin in 1904. It was called the “plum pudding” model because the negative electrons represented the plums embedded in the uniform positive charge, representing the pudding.

1897 J.J. Thomson (continued)
1910 robert a millikan
Millikan, a U.S. physicist, set up an experiment to find the amount of charge of an electron. He already knew Thomson’s hypothesis that the mass of an electron is at least 1000 times less than the smallest atom.

Millikan used an oil drop apparatus to measure the electron’s charge. Oil droplets were sprayed into a chamber by a perfume atomizer. Some droplets fell through a pinhole into a middle chamber, ionized by x-rays, with a positive plate on the top and a negative plate on the bottom. Particles that captured electrons floated upward or fell slowly because they were attracted to the positive plate.

Millikan watched how fast the rise and fall of the drops were and calculated the charge of the drop. He used Thomson’s charge-to-mass ratio of an electron and his own results to calculate the mass of an electron.

1910 Robert A. Millikan
1910 robert a millikan continued
1910 Robert A. Millikan (continued)
  • Millikan won the 1923 Nobel Prize for his work with this experiment and for determining the value of Plank’s constant.

Millikan’s diagram from his Physical Review paper

1911 ernest rutherford
1911 Ernest Rutherford
  • Rutherford, physics professor and former student of Thomson, established the idea that all the positive charge and mass in an atom is contained in a region he called the nucleus with his new nuclear atom model. It was first suggested that an atom has a central nucleus by Japanese physicist Hantaro Nagaoka in 1904.
  • In his experiment, a beam of alpha particles was aimed at a thin sheet of gold foil that was surrounded by a fluorescent screen. Most of the particles passed through the gold atoms with no deflections, but some bounced off the foil at large angles and some bounced straight back.
  • At this time it was believed that the particles should have easily passed through with small deflection because of the positive charge that was believed to be spread throughout the atom.
  • Because of the lack of deflection from most of the particles, Rutherford stated the idea that the atom is mostly empty space. He said that the great deflection of some particles was due to them hitting the positive charge in the central region of the atom he called the nucleus.
1911 ernest rutherford continued
Rutherford’s model of the atom is called the nuclear atom in which protons and neutrons are in the nucleus and electrons take up most of the atom’s volume around the nucleus.1911 Ernest Rutherford (continued)
1913 niels bohr
1913 Niels Bohr
  • Bohr, a Danish physicist and student of Rutherford, proposed the idea that electrons were only found in specific orbits around the nucleus of an atom.
  • Bohr changed Rutherford’s model to include new information about how an atom’s energy changes when light is emitted. Bohr knew that elements gave off color when heated and when passed through a prism, only specific lines or line spectra can be seen. This showed him that atoms could only emit energy in precise quantities. The light that was emitted was due to the electron movement in the atom and Bohr, as well as Rutherford, suggested that electrons can only move in precise steps in the atom.
  • Bohr proposed the idea that electrons occupy energy levels in the atom and when the atom is excited, such as during heating, the electrons can move to higher energy levels. This amount of energy required to move an electron to another level is called a quantum of energy.
1913 niels bohr continued
1913 Niels Bohr (continued)
  • Bohr also hypothesized that each energy level had a certain limit to the number of electrons it could occupy. He said that the maximum number of electrons the first electron shell could hold is two electrons. Elements with more than two electrons would have the extra electrons on the additional electron shells.
  • Bohr won the Nobel prize of 1922 for his work on

the structure of the atom.

1924 louis de broglie
1924 Louis de Broglie
  • Broglie, a French physicist, developed the theory of electron waves.
  • Broglie knew that Einstein had previously proposed that atomic matter may have the same properties as waves. Experiments had also been made which showed that there were restrictions on electron movement because they needed to move around the nucleus. Waves are confined within boundaries by a nuclear charge and are restricted in their motion and shape. If there was an interference, they would be canceled out. Broglie compared this definition of a wave to an electron, which also had restricted motion.
  • Broglie’s theory had no experimental evidence, but offered an explanation to the questions raised by the calculation of electron motion in the atom.
  • Ernest Schrodinger constructed a mathematical system based on de Broglie’s theory. In 1927, Clinton Davisson, Lester Germer, and George Thomson found the first experimental evidence of the similarity between the properties of the wave and electron.
1926 erwin schr dinger
1926Erwin Schrödinger
  • Schrödinger, an Austrian physicist, developed and solved a mathematical equation on electron behavior in a hydrogen atom, using new experimental results and theoretical calculations that went against previous descriptions of the movement of electrons.
  • His mathematical model was based on the combination of the behavior of waves and the de Broglie equations to result in an equation for the electron distribution in an atom. In this model, called the quantum mechanical model, the allowed energy an electron could have and the chances of finding it in different locations in the atom around the nucleus are determined. It shows the probability that an electron can be found in a given location in space at a given time.
1926 erwin schr dinger continued
1926Erwin Schrödinger (continued)
  • Unlike Bohr’s one-dimensional model, Schrödinger’s three-dimensional model required three quantum numbers or coordinates to show the atomic orbitals in which the electrons were found. His model also does not involve the idea of an electron taking a specific path around the nucleus.
  • These three coordinates in his wave equation were the principal (p), angular (l), and magnetic (m) quantum numbers which show the size, shape, and orientation in space of the atomic orbital.
  • This led to Schrödinger’s electron cloud

model which shows that there is a higher

probability of finding an electron in the

cloud’s denser regions.

1932 james chadwick
1932 James Chadwick
  • After Rutherford’s discovery of the positively charged nucleus, it was not determined where the extra mass was in the atom.
  • Chadwick identified the neutron in an experiment in which alpha particles were smashed into beryllium, which released radiation that hit hydrogen atoms in paraffin wax. Protons were released from the paraffin. Chadwick stated that only a particle as heavy as the proton would allow it to be released. He said it had no charge because it easily penetrated the nucleus. These neutral particles were named neutrons by Chadwick.
  • After Chadwick’s discovery, scientists

began the bombardment of neutrons

with many different materials.

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