Thomson, Rutherford, and Bohr Atomic Models
The Thomson Atomic Model
The Plum Pudding Model, also known as the raisin bread model, is a theory about atomic structure proposed in 1904 by Joseph John Thomson, discoverer of the electron. This was before the discovery of the proton and the neutron. In this model, the atom is composed of negatively charged electrons embedded in a positive “soup,” like raisins in a pudding. It was initially thought that electrons were distributed evenly throughout the atom. Later variations postulated a cloud of positive charge. Thomson received the 1906 Nobel Prize in Physics for his discovery. Given that the atom is a material system containing a certain amount of internal energy, this causes a degree of vibration of the electrons in the atomic structure. From this point of view, Thomson’s atomic model can be interpreted as a dynamic model, a consequence of the mobility of electrons within the structure.
From a macroscopic point of view, one can also consider a static structure, since the electrons are immersed and trapped within the mass that defines the positive charge of the atom. This model was superseded after Rutherford’s experiment, which led to the discovery of the atomic nucleus.
The Rutherford Atomic Model
The Rutherford Atomic Model is an atomic model or theory about the internal structure of the atom proposed by the British chemist and physicist Ernest Rutherford to explain the results of his gold foil experiment, held in 1911. Previously, physicists accepted that electrical charges in an atom were distributed more or less uniformly. Rutherford tried to observe how alpha particles were scattered by atoms in a thin gold foil. The angles of deflection of the particles would provide information on how the charge was distributed within the atoms. Specifically, it was expected that if the charges were distributed according to Thomson’s model, most of the particles would pass through the foil, suffering only very slight deflections in an approximately straight path.
Although this was true for the majority of alpha particles, a significant number experienced deflections of about 180°, meaning they practically bounced back in the opposite direction. Rutherford realized that this fraction of particles bouncing back could be explained if it was assumed that there were high concentrations of positive charge in the atom. The Rutherford model was soon replaced by Bohr’s model.
The Bohr Model
The Bohr Model, or Bohr-Rutherford Model, is a quantized model of the atom that Bohr proposed in 1913 to explain how electrons can have stable orbits around the nucleus. This planetary model is a functional model; it does not represent the atom (physical object) itself but explains its behavior through equations. Niels Bohr based his model on the hydrogen atom. Bohr sought to create an atomic model capable of explaining the stability of matter and the discrete emission and absorption spectra observed in gases.
He described the hydrogen atom as having a single proton in its nucleus, with an electron revolving around it. Bohr’s model was conceptually based on Rutherford’s model and the emerging ideas about quantization that had arisen a few years earlier from the investigations of Max Planck and Albert Einstein. Because of its simplicity, the Bohr model is still frequently used as a simplified representation of the structure of matter. Bohr further assumed that the angular momentum of each electron was quantized and could only vary in whole fractions of Planck’s constant. He calculated the distance from the nucleus to each of the allowed orbits in the hydrogen atom, according to the principal quantum number.