Thomson’s model of the atom | Rutherford’s alpha particle scattering experiment | drawbacks in ford

Dalton’s atomic theory suggested that atom was indivisible – which could not be broken down into smaller particles. But the discovery of subatomic particles such as electrons and protons inside the atom disproved this postulate of Dalton’s atomic theory. The discovery of electrons and protons suggested that atoms are divisible and they do have an inner structure. After the discovery that atoms contain electrons and protons, it became necessary to find out how these electrons and protons were arranged inside the atom (Please note that the third subatomic particle neutron had not been discovered at that time). J.J. Thomson was the first scientist to propose a model for the structure of atom When J.J. Thomson proposed his model of the atom in 1903, then only electrons and protons were known to be present in the atom. According to Thomson’s model of the atom : 1. An atom consists of a sphere (or ball) of positive charge with negatively charged electrons embedded in it. 2. The positive and negative charges in an atom are equal in magnitude, due to which an atom is electrically neutral. It has no overall positive or negative charge. The coloured area in the sphere (or ball) contains all the positive charge in the atom. The negatively charged electrons are spread throughout the positive charge. The total negative charge of electrons is equal to the total positive charge of the sphere. These equal and opposite charges balance each other due to which an atom becomes electrically neutral on the whole. Thomson’s model of the atom is similar to that of a Christmas pudding. The electrons embedded in a sphere of positive charge are like the currants (dry fruits) in a spherical Christmas pudding. The red, edible part of watermelon represents the sphere of positive charge whereas the black seeds embedded in watermelon are like the electrons. Although Thomson’s model of atom explained the electrically neutral nature of atom but it could not explain the results of various experiments carried out by other scientists such as Rutherford’s experiments. Before we describe Rutherford’s experiment which led to the discovery of a positively charged nucleus in the atom, we should know the meaning of alpha particles (also written as α-particles). Alpha particle is a positively charged particle having 2 units of positive charge and 4 units of mass. It is actually a helium ion, He2+. Alpha particles are emitted by radioactive elements like radium and polonium. The fast moving alpha particles have a considerable amount of energy. They can penetrate through matter to some extent. We will also be using a gold foil in this experiment. A gold foil is a very thin sheet of gold. Alpha particles are much smaller than the gold atoms present in the gold foil. After the discovery of electrons, protons and neutrons, it became clear that an atom is made up of these three subatomic particles. Experiments were then carried out to find out how electrons, protons and neutrons were arranged in an atom. It was Rutherford’s alpha particle scattering experiment which led to the discovery of a small positively charged nucleus in the atom containing all the protons and neutrons. When fast moving alpha particles are allowed to strike a very thin gold foil in vacuum, it is found that Most of the alpha particles pass straight through the gold foil without any deflection from their original path Gold foil is made up of atoms. If the atoms were solid throughout their volume, then every alpha particle striking them should have changed its path and got deflected. Since most of the alpha particles pass straight through the gold foil without any deflection, it shows that there is a lot of empty space in the atom. 2. A few alpha particles are deflected through small angles and a few are deflected through large angles. We know that similar charges repel each other. So, a positively charged body will repel another positively charged body. The observation that some of the alpha particles are deflected through small and large angles shows that there is a ‘centre of positive charge’ in the atom which repels the positively charged alpha particles and deflects them from their original path. This centre of positive charge in the atom is known as nucleus. Thus, the scattering of alpha particles by a thin gold foil shows the existence of a positively charged nucleus in the atom 3. A very few alpha particles completely rebound on hitting the gold foil and turn back on their path (just as a ball rebounds on hitting a hard wall). A very few of the alpha particles are turned back on their path. This fact cannot be explained only on the basis of repulsion due to positive charge of the nucleus. It can, however, be explained by assuming that the nucleus is very dense and hard.