Thomson's and Rutherford's Different Idea of the Atom

When John Thomson discovered electron the idea of the indivisible atom crashed. All because electrons emerged to be constituents of atoms. A new model of the world of the micro scale was needed.

The scientist who created such model was nobody else but the discoverer of electron - Joseph John Thomson. He came to the conclusion that each and every atom consisted of a homogenous sphere of positive electricity. Inside the sphere were negative electrons. To explain it more precisely how the Thomson's atoms looked like we will consider the atom having three electrons. These electrons (inside the sphere) must be placed symmetrically - in the vertex of an equilateral triangle. When the electrons are resting, the forces of repulse have to be counterbalanced by the attraction of the positively charged sphere. The distance between the resting electrons and the sphere centre should be equal 0,57 of the sphere radius to counterbalance the forces. But if the electrons rotate, one more force arises - the centrifugal one. As the velocity grows the electron's distance from the centre increases until the moment when the electrons leave the sphere. If the velocity still grows the electrons would rotate on the orbits around the sphere to finally flow away (the atom "blows up"). So giving enough energy to the atom (that energy would be changed into kinetic energy of the electrons) one causes its disruption.

Then Thomson considered atoms of more and more electrons. They had to be placed in the way which would ensure stability. For the four particles the placement of the counterbalance is when each particle is placed in one corner of the regular tetrahedron. So the electrons would be situated symmetrically on the surface of a sphere which is concentric to the sphere of the positive electricity. But such system is stable only when the number of electrons is small (not more than seven or eight). If there were more negative particles they would divide into two groups situated on the surfaces of the two bodies concentric to the sphere of the positive electricity. If still increasing the number of electrons in the atom the particles would divide into three groups, then four etc. Thomson came to the conclusion that the "increasing" problem became too complicated for calculations.

Thomson model of the atom was called the plum pudding.

The plum pudding model wasn't a long lasting one. In 1909 Ernest Rutherford and his students conducted an experiment showing that the idea was wrong. The experiment consisted in directing a beam of alpha particles (they have the positive charge) on a thin gold foil. When passing through the foil particles were scattered in different directions because of the influence of Gold atoms. The scientists determined directions of scattering. Of course Thomson theory predicted scattering but it emerged that the prediction was not in agreement with data from the experiment. The most unexpected was the fact that some number of particles was deflected by the gold foil and moved in the opposite direction than at the beginning. The “plum pudding" model didn't deal with such phenomenon. In that model positive charge was just to regular distributed to influence an alpha particle so strongly to repulse it. Something was wrong with that plum pudding!

In 1911 Rutherford made a theoretical analysis of angles of scattering in accordance with Thomson's theory of the atom and in accordance with his own theory. He assumed that atom consisted of positively charged nucleus and negatively charged electrons circling around the nucleus. Then his theoretic calculations he compared with the experiment result. A slow alpha particle falling on the atoms constructed in accordance with the plum cake model (such atoms should cling to each other) would be stopped. So it couldn't go through the foil. And fast alpha particles penetrating the thin foil would be almost not at all deflected, because the electric field inside the atoms would be weak and quite homogenous. In the model created by Rutherford the field is much stronger near to the nucleus, so some of the alpha particles are much more deflected. Other, moving in far distance to the nucleus are almost not at all deflected. The probability that any alpha particle will hit the nucleus is small but there is such a chance.

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Experimental data proved the "planetary" model of the atom. So the atom consists of a nucleus (of size 10-15 to 10-14 meter). The nucleus contains the whole positive charge and almost the whole atom's mass. Around the nucleus, on the area of the order of 10-10 meter, light electrons are rotating. Electrons have to rotate on orbits, not to fall down on the nucleus. The nucleus is over 10000 times smaller than the whole atom is. Almost whole mass of the atom is however gathered in the nucleus. That is over 99,9%!

The Rutherford model of the atom was called the "planetary" one because according to it electrons move around a nucleus just like planets move around the sun. It explained the experimental data very well but still had some serious discordance. According to the classic science, an electron moving around the nucleus should emit an electromagnetic wave. That kind of emission is connected with the escape of some energy from the electron-nucleus configuration. As the energy of an electron decreases, the distance between the electron and the nucleus decreases too. So electrons shouldn't move round but spiral and finally collide with the nucleus. Anyway such phenomenon wasn't observed. Atoms of the most elements are stable. Other discordance regarded the radiation emitted by an atom- this problem we will discuss on the next page. Why doesn't a nucleus disintegrate although it has a very gig charge was also a serious question.

As we see the model of atom created by Rutherford couldn't be the conclusive model of matter's constitution.