History of the Atomic Model

History of the Atomic Model

Just as we once believed that the earth was flat, our understanding of the atom was once extremely limited. As time passes, we begin to discover more and more about the world we live in. We have come to know much about the atom over the past two hundred years through the work of numerous brilliant scientists. Throughout history, scientists have come up with many experiments and atomic models to explain the atoms all around us, all leading up to our modern understanding.
The Greek philosopher Democritus (460 BC- 370 BC) was first to infer the existence of atoms. Democritus believed that atoms were both indestructible and indivisible. On the other hand, Aristotle, another ancient philosopher from that era, had beliefs that were much more widespread at the time, thinking that all things were infinitely divisible. Long after these philosophers came a more modern understanding of the atom, starting with the ideas of John Dalton. In the early 1800's, Dalton spawned ideas that atoms are solid, indestructible spheres unique to each element. Dalton also believed that atoms combine evenly in compounds (all small, whole numbers) and that reactions are rearrangements of atoms. Following Dalton were the discoveries of J.J. Thomson in 1897. Thomson conducted the cathode ray tube experiment, which involved passing electric currents through gasses at a low pressure. From this, he found atomic particles that were repelled by a negatively charged magnet and moved toward the positive magnet, leading to the discovery of negatively charged electrons. From this experiment, he was also able to conclude that electrons have mass based on how they swirled in the tube. The first identification of a subatomic particle inspired Thomson to create his plum-pudding model. This model featured the atom as a positively charged, solid sphere with randomly distributed electrons throughout it like chocolate chips in a chocolate chip cookie. However, the discoveries of Thomson were short-lived thanks to the discoveries of Ernest Rutherford in 1911. Rutherford wanted to test the existing plum-pudding atomic model. To do this he used alpha particles, which were known to be positively charged radiation with a lot of mass. In his experiment, Rutherford shot a narrow beam of alpha particles at a piece of gold foil. If the findings of Thomson were true, the beam would shoot straight through the gold foil with little or no deflection due to the positive charge spread throughout the gold atoms. What Rutherford came to find was that the beam almost always went straight through the gold foil or with slight deflection. To his surprise a very small number of particles, about one in 20,000, shot back at a very large angle. Rutherford remarked, “This is almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you!” Thanks to this experiment, Rutherford suggested a new model of the atom. In his model, the atom was composed of mostly empty space. This explains why most alpha particles shot through the gold foil. But this empty space was accompanied by a positively charged subatomic particle, the proton, which would account for the alpha particles that were deflected. Rutherford then used his findings as well as previously existing ideas about the atom to create an atomic model featuring electrons that move around a nucleus like planets move around the sun. While Rutherford’s atomic model explained few simple characteristics of the atom, it failed to explain the chemical properties of elements. In 1913, a student of Rutherford’s named Niels Bohr reinvented the atomic model. Bohr proposed that electrons are arranged in concentric orbits around the nucleus. Because his model was patterned on the solar system it was named the planetary model. Bohr’s model had four principles. The first principle was that electrons occupy only certain orbits around the nucleus that are both stationary and stable. The second principle stated that each orbit has an energy associated with it. The orbit nearest to the nucleus was E1, the next being E2 and so on. The third principle said that energy is absorbed when an electron jumps from a lower to a higher one and energy is emitted when an electron falls from a higher orbit to a lower one. Lastly, the fourth principle was that the energy and frequency of light emitted or absorbed can be calculated by using the difference between the two orbital energies. However, Bohr’s model was soon also disproved after new understanding of electrons found that they have both particle and wave-like properties. In 1926, Erwin Schrodinger took the Bohr atomic model one step further while also taking it in a new direction. Schrodinger used mathematical equations to describe the likelihood of finding an electron in a certain position. The quantum mechanical model, also known as the cloud model, did not define the exact path of an electron, but instead predicted the probability of the location of the electron. Schrodinger’s model can be portrayed by a nucleus surrounded by an electron cloud, or dot-like points scattered all around a central nucleus. Where the cloud is most dense, the probability of finding the electron is greatest, whereas the least dense areas of the cloud represent a lower probability of finding the electron. Thus, Schrodinger’s model introduced the concept of sub-energy levels. In addition, the patterns found in Schrodinger’s model coincided with Bohr’s model. The next breakthrough in the atomic model was due to the findings of James Chadwick in 1932. Chadwick had been convinced of the existence of neutrons for 12 years before his discovery in 1932, and eventually earned a Nobel Prize for his work in 1935. By 1910, it was known that the atom consisted of a massive nucleus made of protons with orbiting electrons, but measurements of atomic mass showed that all nuclei needed to contain integer numbers of another particle. Along with this, if nuclei only contained protons, their charge would be much higher. Chadwick knew that there had to be another subatomic particle in the atom to account for the missing mass and the difference in the charge. Rutherford and Chadwick worked together for some time and concluded that a so-called ‘neutral doublet’ could be responsible for both mass and charge. In 1930, German physicists Bothe and Becker shot alpha particles at Beryllium and noticed that radiation was emitted. They assumed the radiation was gamma rays because it was non-ionizing. In 1932, Irene and Frederic Joliot-Curie investigated this radiation in France. They used the radiation to hit a block of paraffin wax, and found the wax to emit protons. By measuring the speeds of these protons, they found that the gamma rays would have to be ‘incredibly energetic’ to knock them from the wax. Chadwick told Rutherford of Joliot-Curie's experiment, who did not think that gamma rays could account for the protons from the wax. Both Rutherford and Chadwick were convinced that the Beryllium was emitting neutrons. To test this, Chadwick shot alpha particles at Beryllium, which then transformed into carbon and freed one neutron. Bothe and Becker had thought these neutrons were gamma rays, but in 1932 Chadwick finally identified them as neutrons. This was called the wax project. Chadwick’s discovery of neutrons soon after helped to develop nuclear power and nuclear weapons in World War II. Since the work of these scientists, we have come to better understand the atom. We have come to know the atom to be made up of three subatomic particles. The electron, which was first discovered by Thomson, is now thought to have multiple orbitals around a central nucleus. Different electron probability clouds can form when there are multiple electrons. The nucleus is composed of protons, discovered by Rutherford, and neutrons, discovered by Chadwick. Neutrons are no longer thought to be composed of a combination of protons and electrons as Chadwick had thought. Now we believe there are even smaller particles that make up subatomic particles called quarks, which are thought to be vibrating strings of energy. We may be proved wrong in the future, as science is always advancing, or we may finally have it right thanks to the work of many brilliant scientists: including Dalton, Thomson, Rutherford, Bohr, Schrodinger, and Chadwick. We would not understand the world around us as we do without them.