The First Atomic Theory The history of the atom began in around 460 B.C, when a Greek philosopher named Democritus came up with the first atomic theory after asking if matter could be divided infinitely or if there is a limit to the number of times something can be divided into. He finally theorized that if matter is divided a certain number of times, the smallest object possible would be obtained, which is indivisible. Democritus named this particle "atomos," meaning "cannot be divided." He thought these particles consisted of the same material, but could be found in different shapes and sizes. These particles can join up to form matter. Unfortunately, his theory was never considered seriously for a long time, as most Greek philosophers, including Aristotle, opposed his idea.
More Influential Discoveries and Theories
For more than 2,000 years, Greek sciences regarding matter was lost. People never seriously considered the structure of things in the world until the 1800s. The first influential scientist supporting the atomic theory since the Greeks would emerge during this time, and his name is John Dalton. Dalton was born in 1866, and grew up to be a great chemist. As he was experimenting with chemicals, which seemed to prove that matter consisted of tiny particles--atoms. He concluded several things about atoms. He stated that all atoms are indivisible as well as immutable, all atoms of the same element are the same, all atoms can fuse to form compounds, all chemical reactions are the results of rearranging atoms, and that atoms cannot be created. He influenced many more scientists to continue exploring atoms.
The next great atomic theorist is J.J. Thomson, an English physicist. He completely changed previous thoughts of the structure of an atom by discovering electrons. He proposed the existence of electrons after wondering how patterns from electric currents were caused. Thus, he proved that atoms were divisible because they contained smaller particles within. His atomic model is known as the Plum Pudding Model, and it portrayed a positively-charged ball entwined with electrons around it. His discovery is one of the most significant ones in atomic history, and will be the foundation for more atomic innovations in the future.
Fourteen years later in 1911, Ernest Rutherford prove all theories regarding the structure of the atom incorrect. Previous thought of the atom assumed that it was structured as a sphere without any complex structures, but Rutherford found that most of an atom was just space with nearly all of its mass in a small concentrated ball in the center. Two years later he would find that the nucleus of a helium atom takes up a billionth of its total volume although it contained virtually all of the mass in an atom. After, he realized that the electrons existed near the outside of an atom, and he theorized that electrons orbited the nucleus much like planets orbiting the Sun. In 1919, Rutherford also found that the nucleus could contain multiple positively-charged particles, and he called these protons. He thought the charge protons and electrons were equal in intensity.
However, Rutherford's model was opposed by many because particles of different charges were attracted to each other, which does not allow electrons to orbit protons. In 1912, Neil Bohr created a refined model of Rutherford's atomic theory, and proposed restrictions upon laws of matter that only applied to atoms. He said that electrons with different energy levels orbit the nucleus in unique and constant paths from the nucleus. He also theorized that electrons orbit the nucleus without ever losing energy, and that low-energy electrons orbit close to the nucleus and high-energy electrons orbit farther away from the nucleus. However, Bohr's idea proved to only be accurate when it came to atoms with one electron and did not solve many questions regarding Rutherford's model, including why the electrons did not get sucked into the nucleus because opposite charges attract. Although the most recent atomic theories regarding the structure of atoms are a bit different from Bohr's model, it is typically used by many today. He explained much about atoms, but of course, there is always more.
The next significant event in atomic studies was the discovery of the neutron by James Chadwick in 1932. Originally, Rutherford thought that there was a particle with no charge but large mass, but there was little proof and the idea was rejected. The neutron has similar mass to a proton and is located in the nucleus along with protons. With his new discovery, Chadwick could now answer the question, "How can two atoms of the same element have identical properties but one is radioactive and the other is not?" These particles are known as isotopes, and they are defined as particles of the same element but with a different number of neutrons.
After Bohr came Schrodinger, an Austrian physicist who formed the Schrodinger Equation and the Wave Model. Although it shared the same concept of the nucleus and electrons with different energy levels, there were several key differences. For example, Schrodinger's Wave Model portrays electrons without a fixed path, but with certain areas where they are most probable to be. Schrodinger stated that the location and momentum of an electron cannot be pinpointed at a given time, but can be estimated. To him, electron paths could be seen as a "cloud" with the densest parts of the cloud as where electrons were most likely to cross. His model explained many issues that Bohr's model could not solve, and worked with almost all elements, unlike Bohr's model.