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> The evolution of the atomic model Issue: 2007-3 Section: Science



The object of this paper is the description of the matter structure through the enumeration of the major part of the theories formulated about it, in the last centuries. To understand the elementary constitution of the matter the experiments executed by the scientists through the time will be cited.


Since ancient times man tried to give some answers to the question regarding the constitution of the matter: What is it composed of?, How far is it possible to subdivide it?

The ancient Greeks had two different theories: some affirmed its endless divisibility, others that it was composed by elementary units. These were only philosophic theories, without any reliable scientific investigation. On the bases of this last theory some philosophers suggested some models, among them, Democritus, who affirmed that all the matter was constituted by an infinity of indivisible units, the atoms (which in Greek means indivisible), and that the atoms had no qualitative property taste, smell, color, but were characterized by quantitative property, such as dimension, shape and position.

The atomistic hypothesis of Democritus was then abandoned, but it has been revisited in recent times.


Modern models

1. Rutherford's experiment

2. Rutherford's model


1. Bohr's atomic model

The study of the matter, with a modern scientific method, was started by Antoine Lavoisier with the law of the conservation of the mass, followed by Joseph Proust with the law of the definite proportions and by John Dalton with the law of the multiple proportions and with his atomic theory (1803), which asserted that the matter was composed by a great number of indivisible particles, that is the atoms, among which, those of the same element are all equal and have the same mass and cannot be created, nor destroyed.


With the discovery of the X rays, in 1895 by William Roentgen, and with the photoelectric effect of Albert Einstein, the scientists realized that they had to review the atom’s concept. In 1904 J. J. Thomson through the study of the cathode rays found out the electrons, that is little particles with very small mass and negative charge, which was calculated by Robert Millikan, and also other particles of opposite charge and major mass.

On the basis of these results in 1906 Thomson suggested that the atom wasn’t indivisible, but constituted of a positive spherical structure into which the electrons where equally displaced so to render altogether the atom neutral.

Thomson’s model was contradicted by the experiments carried out by Ernest Rutherford, and by his students, Geiger e Marsden, with the alpha particles. In fact, Rutherford, bombarding with these particles a thin gold sheet, observed that the greatest part of them maintained the direction of the start, some were lightly deviated and a very small percentage was repelled. If Thomson’s model had been correct, the same deviation for every particle would be noticed, because, in his opinion, the positive and negative charge should be disposed in a homogeneous way into the atom.

Following these data, in opposition with Thomson’s model, Rutherford supposed that the atom was made of a nucleus, much more smaller than the atom in which the major part of the atomic mass and the positive charge (justifying the deviation of some alpha particles because of the repulsion ) resides, and of the electrons which turn around quickly to maintain their own balance, between the attraction exercised by the nucleus and the reciprocal repulsion.


In 1920 he gave to the atomic nucleus the name of proton, affirming that it could be formed by more protons and in each atom the number of protons and electrons should be equal. This model belied the laws of the classic electromagnetic theory, because the electron has a charge and therefore, being accelerated, it should irradiate energy, falling in few instants on the nucleus, a phenomenon which would involve the emission of all its frequencies in the passage from its level to the nucleus. These hypotheses cannot be proved in reality in that the atoms are stable, because they don’t have a fixed emission frequency in the spectrum.

In 1913 Niels Bohr improved Rutherford’s model taking cue from the results of Max Planck and Albert Einstein. The former introduced, out of the classic physics, the concept of quantization, that is a quantity can take only certain permitted values and the energy isn’t emitted in a continuous way, but in quanta; the latter extended the concept of quantum to the light, asserting that it was composed by quanta, that is the photons. Altogether Bohr’s atomic model was based on two principles: the quantization of the orbits, that is the electron could occupy only determinate orbits, and the quantization of the energy, that is when a electron covers a certain orbit it doesn’t emit or absorb energy, except in the case in which the electron passes from an orbit to an other.

This model had some disadvantages because it couldn’t be applied to the atoms with more than one electron and could not explain the distribution of the electrons into the orbits. Bohr, in fact, always handled the electrons as classic particles to which he could apply the laws of the mechanic.


In 1925 there was a turning-point with the theory formulated by Louis De Broglie, who began to consider the electron with corpuscular and undulated property; this theory was verified experimentally by Davisson and Germer, bombarding a crystal of nickel with a band of electrons and obtaining the same diffraction, which was observed in the X rays (this double property to the radiation had just been assigned them). These new discoveries brought about the rise of quantum physics, from the moment that the wave-particle duality was given to the matter.

In 1926 Erwin Schrodinger formulated a mathematic equation, which described the behavior of the electron as a wave, into which, following Max Born’s opinion, the square of the absolute value of the wave width of a electron represented the probability to find the electron in a point of the space around the nucleus. In this way the electron is represented only as a wave, which behaved as a particle.

Since then the concept of the orbit was abandoned and it was substituted by the concept of orbital, that is the region of space into which it is more probable to find an electron around the nucleus.

One year later Werner Heisenberg developed the knowledge of the atomic structure, noticing that it wasn’t possible to know at the same time the position and the exact moment of an elementary particle, because of the principle of indetermination.


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