Around 400 BC, a Greek philosopher named Democritus suggested the first atomic theory, explaining that all things are "composed of minute, invisible, indestructible particles of pure matter which move about eternally in infinite empty." Although at that time there was no technology to research Democritus' theory of the atom, he was surprisingly accurate. After Democritus' discovery, new evidence would be found that would eventually lead to the modern atomic theory. John Dalton, J.J. Thompson, Ernest Rutherford, Niels Bohr, James Chadwick and Ernest Schrodinger each contributed greatly to the modern atomic theory by finding the actual evidence.
John Dalton, a British schoolmaster and chemist, made inferences in the 19th century that exhibited how atoms bond together in definite proportions. An example of one of the tests that he ran was measuring the proportions of hydrogen to oxygen in different amount of the substances. Because the ratios were the same, Dalton was able to say that atoms of different elements combine in whole number ratios. This theory, to go along with four other theories, made up what Dalton called the "Modern Atomic Theory." In this, Dalton stated five different theories that he believed were true of atoms. Included in these were two theories that stated atoms could not be divided or destroyed, a theory that stated different elements contain different chemical properties, and atoms of the same element contain the same chemical properties. Although the two theories that speculated atoms couldn't be divided were false, Dalton contributed greatly to the advances of atomic theory, and would greatly influence J.J. Thompson in his own discoveries.
J.J. Thompson, the person who is credited for discovering the electron, ended up contributing to more than the atomic theory. The television, computer, and radar are machines that he indirectly helped create. Thompson created a tube that had a positively charged anode on one side and a negatively charged cathode on the other side. Thompson then applied a magnet to the middle of the tube, equal distance from the cathode and anode, and discovered that negatively charged particles were emanating towards the positive magnetic field. From this, Thompson concluded that negatively charged particles, called electrons, were present in atoms. Thompson then created the Plum Pudding model, which suggested that electrons and protons were randomly placed throughout the atom. This theory wasn't correct, but led to the discovery of the nucleus, made by Ernest Rutherford.
Ernest Rutherford's experiment was to emit alpha particles, which were actually stripped helium atoms, towards a thin gold sheet. Rutherford would then determine where the deflections of the alpha particles would go, and therefore be able to theorize what kind of placement protons and electrons had. The results of the experiments were somewhat revolutionary; most of the alpha particles did not deflect off of the gold foil, and instead went strait through. Only 1/1,000,000 particles were deflected straight back, and just a bit higher percentage were deflected in other angles. Rutherford then theorized that there was something called a nucleus, which contained a high density of positively charged particles. Rutherford was able to say there was a nucleus because alpha particles that deflected right back must have hit something strongly positive. Rutherford then went on to suggest that the atom contained electrons surrounding the nucleus and some sort of positive charge in the nucleus itself.
Niels Bohr, whose "Bohr Theory of the Atom" was the closest to the modern atomic theory re-emphasized the idea of electrons around the nucleus. Bohr suggested that electrons orbited around the nucleus in seven different quantum levels, or shells. The evidence that Bohr used to imply this theory was the measurement of the line spectrum given off by each of the electrons. Bohr determined that different energy levels could be found by using mathematical formulas, which measured the wavelengths of the different energy levels. Bohr went on to suggest that electrons would only occupy the lowest possible energy level on the respective level they were on. Furthermore, electrons would only move up a level (increasing energy) if the lower levels were full. Bohr's model was very accurate, and would lead to Schrodinger's idea of modern atomic model.
Erwin Schrodinger's most important contribution to the modern atomic theory was his development of the mathematical description that described the paths electrons would most likely follow in their orbits around nucleus. The formulas that Schrodinger developed in 1926 would be later called the basis of quantum mechanics, and awarded him a Nobel Prize. Eventually, Schrodinger determined that instead of Bohr's suggested orbits, there were actually orbitals. Instead of the idea of the electrons following a pre-determined path, the electrons would be moving around in an area. These ideas, including the quantam mechanical formulas, were presented in his "Wave Mechanical formula." This model eventually became the basic modern atomic theory.
Scientists every day make new discoveries pertaining to the atomic theory, and as new evidence is introduced, new ideas and theories will be suggested. Hopefully, in the years to come, the world will see more people like those who helped lead to the modern atomic theory, making grounds for a better understanding of atoms.