After the common occurrence of the electrons - the more basic particles than atoms - had been proved and when the phenomena of radioactivity were discovered, Dalton's theory of the indivisible atom fell down. Basing on the researches of polonium and radium lord Kelvin formulated a model of their consistence. He assumed that every particle was more or less radioactive. He also assumed that radium and polonium were different because of the high speed of the radioactivity proceeding. The diagram of the polonium atom according to Kelvin's theory is shown on the picture below:
Such particle was to consist of four elementary, negative charges (-4*e) connected with another four ones by one positive charge. The positive charge was to be placed half-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 to -8*e,and of the positive charges only to 1*e. To be neutral the structure should have seven more positive charges (placed all over the particle). Those seven charges were to be emitted during the radioactive disintegration.
Thomson developed the idea of Kelvin. He 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 to 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 be counterbalanced by the attraction of the positively charged sphere. The distance between the resting electrons and the sphere centre should be equal to 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 be placed in such way to assure the stability. For the four particles the placement of the counterbalance occurs when each particle is in one corner of the regular tetrahedron. So the electrons would situate 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 (no more than seven or eight). If there were more particles they would divide into two groups situating 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 divide into three group, then four etc. Thomson came to the conclusion that the "increasing" problem became too complicated for calculations.