Plum pudding model

The Thomson model of the atom is an atom model, according to which the atom consists of a uniformly distributed, positively charged mass in which move the negatively charged electrons. It was developed in 1903 by Joseph John Thomson. Due to the assumed arrangement of the electrons in the ground, similar to raisins in a cake, it is also referred to as plum pudding or raisin cake model. In the initial state, the electrons are distributed so that its potential energy is a minimum. If they are excited, they start to oscillate.

Formation

In 1897, Joseph John Thomson demonstrated that cathode rays of charged particles called electrons exist. With a greatly improved vacuum he could determine the ratio of charge to mass. Thomson suggested that the electrons were already present in the atoms of the cathode and presented in 1903 the first atom model on which the atoms attributed to an internal structure.

Construction

In Thomson's original model, the electrons were alone responsible for the mass of each atom. In a hydrogen atom thus already had to be present more than 1800 electrons. However, he also took the existing equally large positive charge amount as massless. The positive charge filled in the volume of the atom and was - except for electrons - impenetrable. Although Thomson knew that in such a structured atom could not be stable, but he was able to quantitatively explain the normal Zeeman effect with it.

Problems and refinement

Experiments with X-rays showed Thomson in 1906 that the number of electrons needed to be significantly lower than predicted by him. Their number could only be equal in an atom as the mass number of the atom.

This results in an electron in the hydrogen atom when it is excited, harmonic vibrations through the center of the atom and sends by light. This allows, in contrast to experimental observation, however, only one spectral line.

The Rutherford scattering experiment (1909 ), conducted by Hans Geiger, Ernest Marsden and Ernest Rutherford showed that the positive charge is united in an atomic nucleus and accounts for most of the atomic mass. With the help of this knowledge, the Thomson model was refined in 1911 to rutherford Atomic Energy Model.