After the common occurrence of the electrons - the more basic particles than atoms - was proved and when the phenomena of the radioactivity was discovered, the Dalton's theory of the indivisible atom fell down. Basing on the researches on polonium and radium lord Kelvin formulated a model of their consistence. He assumed that every particle is more or less but radioactive. He also assumed that radium and polonium are different because of the high speed of the radioactivity proceeding. The diagram of the polonium atom according to the Kelvin's theory is shown on the picture below:
Such particle was to consists of four elementary, negative charges (-4*e) connected with another four ones by one positive charge. The positive charge was to be placed half a way between the groups of the negative charges. Such structure (according to Kelvin) made the particle stable - the Coulomb forces influencing each particle were counterbalanced. The whole range of the negative charges in that structure was equal -8*e,and of the positive charges only 1*e. To be neutral the structure should have seven more positive charges (placed all over the particle). That seven charges were to be emitted during the radioactive disintegration.
Thomson developed the idea of Kelvin. Thomson came to the conclusion that every atom consisted of the homogenous sphere of the positive electricity. Inside the sphere there were to be negative electrons moving on orbits round the sphere centre. The whole range of negative charge collected on the electrons was equal the range of positive charge collected on the sphere surface.
To explain it more precisely how the Thomson's atoms looked like we will consider the atom of 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 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 circulate there appears one more force - 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 circulate 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 placed in such way to assure the stability. For the four particles the placement of the counterbalance is when each particle in one corner of the regular tetrahedron. So the electrons would situate on the surface of a sphere concentric to the sphere of the positive electricity. But such system is stable only when the number of electrons is small (no more than seven or eight). If there were more particles they divide into two groups situating on the surfaces of the two bodies concentric to the sphere of the positive electricity. Still increasing the number of electrons in the atom the particles divide into three group, then four etc. Thomson came to the conclusion that the "increasing" problem becomes to complicated for calculations.
The model was called "the plum cake model".
