We explain what Rutherford's atomic model is and its main postulates. Also, how was Rutherford's experiment.
What is Rutherford's atomic model?
Rutherford's atomic model, as its name implies, was one of the models proposed to explain the structure internal of atom. In 1911 the British chemist and physicist Ernest Rutherford proposed this model based on the results of his experimentation with gold leaf.
This model constituted a break with previous models such as the Dalton atomic model and the Thompson atomic model, and a step forward with respect to the currently accepted model.
In its atomic model, Rutherford proposed that atoms have a central nucleus where the highest percentage of their mass. Furthermore, according to this theory, this nucleus has a positive electric charge and is orbited by particles of opposite charge and smaller size (electrons).
According to his considerations, the atom operated as a Solar system of electrons orbiting a heavier atomic nucleus, as planets do around the Sun.
Rutherford's atomic model can be summarized in the following three propositions:
- Most of the atomic mass is concentrated in the nucleus, which is larger and larger. weight than the rest of the particles, and endowed with a positive electric charge.
- Around the nucleus and at great distances from it are the electrons, with a negative electric charge, that orbit it in circular paths.
- The sum of the positive and negative electric charges of an atom should give zero as a result, that is, they should be equal, so that the atom is electrically neutral.
Rutherford not only proposed this structure for the atom, but also calculated its size and compared it with the size of the nucleus, and came to the conclusion that a good part of the atom's composition is empty space.
This model, on the other hand, has certain limitations that could be resolved with the advancement of the knowledge and the technology:
- It could not be explained how it was possible for a set of positive charges to be held together in the atomic nucleus, since they should repel each other, since they are all charges of the same sign.
- The stability of the atom could not be explained, because when considering the electrons of negative charge that rotate around the positive nucleus, at some point these electrons had to lose Energy and collapse against the core.
Rutherford's atomic model was in force for a short time, and was replaced by the atomic model proposed by the Danish physicist Niels Bohr in 1913, in which some of the limitations were resolved and the theoretical proposals developed by Albert Einstein in 1905 were incorporated.
Rutherford's experiment
Rutherford's experimental method started with several thin sheets of gold that would be bombarded in the laboratory with helium nuclei (alpha particles, which have a positive charge), thus measuring the deflection angles of the particle beam when passing through the gold.
This behavior, which sometimes reached deviations of up to 90 °, did not agree with the atomic model proposed by Thompson, prevailing at the time.
Thompson's model proposes that the atom is a positive sphere, with negatively charged electrons embedded in it. For this reason the model resembles a pudding with raisins: the pudding would be the atom and the raisins would be the electrons.
On the other hand, Rutherford's model states that the atom has the positive charge concentrated in the nucleus and the electrons orbit around it. If the atom had the structure proposed by Thompson, the alpha (positive) particles, when passing through the gold foil, should follow their trajectories or deviate very little. However, what happened is that deviations of these particles of up to 90 and 180 ° were seen, which showed that the atom, indeed, has the positive charge concentrated in its center (as proposed by Rutherford) and not distributed in a sphere. (as proposed by Thompson).