Atomic Structure: From Thomson to Rutherford Models

The Thomson Atomic Model

The Thomson Model was based on the discovery of the electron as a particle of very small mass and negative charge; he proposed that all atoms have electrons. But as the atom is neutral, the rest of the "dough" should be positive. When the protons were discovered, they were found to have the same charge as the electron but positive, and they had the same mass as hydrogen (though this was later corrected).

Rutherford's Nuclear Atom

The Nuclear or Rutherford atom experimentally found that in almost all atoms, the mass is located in a central area called the core or nucleus. This is where the positive charges of the atom (protons) are located, while electrons are encountered orbiting outside the nucleus in circular orbits. When the neutron was discovered (a particle with no electric charge and a mass similar to that of a proton), it was found to lie at the heart of the atom. This was the first model that established the distinction between a nucleus and an extranuclear electronic structure.

When the atom is neutral, the number of protons is equal to the number of electrons in the cortex. This is represented by the letter Z. All atoms of the same element have the same atomic number [Z]; therefore, the value of Z determines the element that owns the atom. Two atoms of the same element may have a different mass number, in which case we say they are isotopes.

Extranuclear Structures and Electron Orbits

In extranuclear structures, electrons revolve around the nucleus in circular orbits. The spokes could be any electrons and could have had the same radius. The forces that keep the electron in the atom are due to the attraction between the negatively charged electron and the positive protons. It was determined by Coulomb experimentally that the force between two charges is directly proportional to the product value of these charges and inversely proportional to the squared distance from them.

The Force and Energy of Electron Orbits

The force acts in the direction that joins the charges; it is an attraction when the charges are of opposite signs and repulsive when they are of the same sign. The greater the radius of an orbit, the higher the energy that an electron has in that orbit; thus, higher electron orbits possess higher energy. When an electron moves from a higher to a lower orbit, it loses energy, and that energy is emitted as radiation in the form of light.

The Spectrum of Radiation

The radiation emitted can be recorded by a plate or film. By disclosing such records, it was encountered that if the radiation came from a few atoms of any element in a gaseous and atomic state, the film only showed some streaks whose distribution was characteristic of each element. The spectrum of radiation refers to all those simple radiations that make up that radiation.