Determination of Charge/Mass Ratio of Positively Charged FragmentsIf one uses a perforated cathode, a beam of particles will be emitted from the back of the cathode. These are the positively charged fragments produced in the discharge tube. The value of e/m for these particles was determined in the same manner as described above and was found to be dependent upon the nature of the gas in the tube.
The highest charge/mass ratio (i.e. the smallest positively charged particle) was obtained when the gas was hydrogen. This particle is now known as the proton and its e/m is found to be 9.57 107 C kg-1. From the above experiments, it was concluded that an atom can be split up into two fragments, namely, (1) negatively charged electrons which are common to all atoms, and (2) the positively charged particle which differs from atom to atom.
Based on this fact, J.J. Thomson, in 1898, put forward his model of the atom. According to him, an atom is a positively charged sphere of radius about 10-10 m in which negatively charged electrons are uniformly embedded so as to have an overall electrically neutral atom.
Atoms are made up of electron, proton and neutron. The proton, a positively charged particle, is present in the highly dense central part of the atom called nucleus. The electron, a negatively charged particle, is present outside the nucleus of the atom. The neutron, a neutral particle is also present in the nucleus of the atom. Since an atom is electrically neutral, the number of electrons present in the atom is equal to the number of protons in the nucleus.