Even in ancient Greece, philosophers guessed about the internal structure of matter. And the first models of the atomic structure appeared at the beginning of the 20th century. The hypothesis of J. Thomson was not taken by the scientific community critically - after all, various theories about what is inside the smallest particles of matter have already been put forward.
Raisin Pudding, or Thomson’s Model
Until the 19th century, scientists assumed that an atom was indivisible. However, everything changed after Joseph Thomson discovered the electron in 1897 - it became clear that scientists were wrong. Both atom models of Thomson and Rutherford were advanced at the beginning of the last century. The first model was W. Thomson, who suggested that the atom is a bunch of matter with a positive electric charge. Evenly distributed electrons are located inside this bunch - that is why this model was called “cupcake”. Indeed, according to it, the electrons in matter are arranged like raisins in a cupcake. Another unofficial name for the model is Raisin Pudding.
Merits of J. Thomson
This model was developed in more detail by J.J. Thomson. Unlike W. Thomson, he assumed that the electrons in the atom are located exactly on the same plane, which are concentric rings. Despite the equal importance of the atomic models of Thomson and Rutherford for the science of that time, it is worth noting that J. Thomson, among other things, was the first to propose a method for determining the number of electrons inside an atom. His method was based on X-ray scattering. J. Thomson suggested that it is the electrons that are those particles that should be in the center of the scattering of rays. In addition, it was Thomson who was the scientist who discovered the electrons. In modern schools, it is with the study of his discoveries that the study of the course of quantum mechanics begins.
Cons of Thomson's Theory
However, compared with the Rutherford model, the Thomson atom model had one significant drawback. She could not explain the discrete nature of the radiation of the atom. It was impossible with her help to say anything about the reasons for the stability of the atom. It was finally refuted when the famous experiments of Rutherford were made. The Thomson atom model was no less valuable to the science of that time than other hypotheses. It must be borne in mind that all of these models, available at that time, were purely hypothetical.
Features of the Rutherford Experience
In 1906-1909, G. Geiger, E. Marsen, and E. Rutherford conducted experiments in which alpha particles were scattered on the surface of a gold foil. Briefly atom models of Thomson and Rutherford are described as follows. In the Thomson model, electrons are distributed unevenly in the atom, and in Rutherford's theory they rotate in concentric planes. A distinguishing factor in Rutherford's experiment was the use of alpha particles instead of electrons. Alpha particles, unlike electrons, had a much larger mass, and did not undergo significant deviations when they collided with electrons. Therefore, scientists had the opportunity to register only those collisions that occurred with the positively charged part of the atom.
The role of Rutherford's discovery
This experience was crucial to science. With its help, scientists were able to get answers to those questions that remained a mystery to the authors of various models of the atom. Thomson, Rutherford and Bohr, although they had the same base, nevertheless made a somewhat different contribution to science - and the results of Rutherford's experiments in this case were amazing. Their results were directly opposite to what scientists expected to see.
Most of the alpha particles passed through the foil sheet along straight (or almost straight) paths. However, the trajectories of some alpha particles deviated at significant angles. And this was evidence that the atom was a formation with a very high density, and had a positive charge. In 1911, based on experimental data, a model of the structure of the Rutherford atom was put forward. Thomson, whose theory was previously considered dominant, at that time continued to work in the laboratory of Cavendish University. Until the end of his life, the scientist continued to believe in the existence of a mechanical ether, despite all the successes in scientific research of that time.
Rutherford Planetary Model
Summarizing the results of the experiments, Ernest Rutherford put forward the main points of his theory: according to it, an atom consists of a heavy and dense nucleus of very small sizes; around this nucleus are electrons in continuous motion. The radii of the orbits of these electrons are also small: they are 10–9 m. This model was called “planetary” because of its similarity to the model of the solar system. In it, the planets move in elliptical orbits around a huge and massive center with attraction - the Sun.
Electrons rotate in the atom at such a gigantic speed that they form a kind of cloud around the surface of the atom. According to Rutherford's theory, atoms are located at a certain distance from each other, which allows them not to stick together. Indeed, around each of them there is a negatively charged electron shell.
Thomson and Rutherford Atom Models: Key Differences
What are the main differences between the two most important theories of atomic structure? Rutherford assumed that in the center of the atom is a nucleus with a positive electric charge, and the volume of which, in comparison with the size of the atom, is negligible. Thomson, on the other hand, assumed that the whole atom is a high-density formation. The second main difference was an understanding of the position of electrons in an atom. According to Rutherford, they rotate around the nucleus, and their number is approximately equal to ½ the atomic mass of the chemical element. In Thomson's theory, the electrons inside the atom are distributed unevenly.
Cons of Rutherford's Theory
However, despite all the advantages, at that time Rutherford's theory contained one important contradiction. According to the laws of classical electrodynamics, an electron orbiting a nucleus should constantly emit portions of electrical energy. Because of this, the radius of the orbit along which the electron moves had to continuously emit electromagnetic radiation. According to these ideas, the lifetime of an atom should be negligible.
Most often, when they talk about the discovery of the internal structure of an atom, they mention the names of Thomson and Rutherford. The experiments of Rutherford, the atomic model of which is now known to every student in physics and mathematics at universities, is now part of the history of science. When Rutherford made his discovery, he exclaimed: “Now I know what an atom looks like!” However, in reality, he was mistaken, because the true picture became known to scientists much later. Although the Rutherford model has undergone significant adjustments over time, its meaning has remained unchanged.
Bora Model
However, in addition to the atomic models of Thomson and Rutherford, there was another theory that explained the internal structure of these smallest particles of matter. It belongs to Niels Bohr, a Danish physicist who proposed his explanation in 1913. According to his model, an electron in an atom does not obey standard physical laws. It was Bohr who was the scientist who introduced the concept of the relationship between the radius of an electron’s orbit and its speed in science.
In the process of creating his theory, Bohr took Rutherford's model as a basis, but he subjected it to significant refinement. The atomic models of Bohr, Rutherford, and Thomson may seem somewhat simple now, but they are the ones that formed the basis of modern ideas about the internal structure of the atom. Today, the quantum model of the atom is generally accepted. Despite the fact that quantum mechanics cannot describe the motion of the planets of the solar system, the concept of orbit still remains in theories that describe the internal structure of an atom.