atomic_model_sonam_2_41_1_13

Atomic Model Thomson’s atomic model Thomson assumed that an electron is two thousand times lighter than a proton and believed that an atom is made up of thousands of electrons. In this atomic structure model, he considered atoms surrounded by a cloud having positive as well as negative charges. Thomson’s model of an atom is similar to a plum pudding. Thomson atomic model was called the plum pudding model. In this atomic model, he considered that atoms are surrounded by a cloud that is having both positive and negative charges. Thomson’s model of an atom was called the plum pudding model also where the atom was visualised as a pudding of positive charge and the electrons as the plum embedded in the pudding. Postulates of Thomson atomic model 1. An atom is considered to be a sphere of uniform positive charge and electrons are embedded into it. 2. The total positive charge is equal to the total negative charge so that an atom as a whole is electrically neutral. 3. The mass of an atom is considered to be uniformly distributed. 2

A watermelon is a common analogy of Thomson’s atomic model where he considered: • Watermelon seeds embedded as negatively charged particles. • The red stuff of the watermelon as positively charged. Merits of Thomson atomic model • The model accounted for the electrically neutral nature of the atom. • The model gave a clue that the mass of the atom is uniformly spread over the atom. Demerits of Thomson’s atomic model The model was not satisfying because it was not useful in predicting and explaining the chemical properties of atoms. • The model failed to explain how a positive charge holds the negatively charged electrons in an atom. Therefore, failed to explain the stability of atom. • The model also failed to account for the position of the nucleus in an atom. • Thomson’s model of the atom had to be abandoned in 1911 when Ernest Rutherford performed the α- scattering experiments. Alpha particle scattering experiment In 1911, Ernest Rutherford, a scientist from New Zealand who conducted an experiment by bombarding a thin sheet of gold with α-particles and studied the trajectory of these particles after their interaction with the gold foil. 3

In this experiment, • The alpha particles are produced by radioactive disintegration of radium in a block of lead. • A narrow beam of these particles comes out from a deep hole in the lead block. • The beam is collimated by a lead silt and directed at thin gold foil, • The alpha particles are detected by the light they produce when they collide with zinc sulphide coated screen. • The entire set up is enclosed in a vacuum chamber so that the deflection of alpha particles are not influenced by the collision with gaseous molecules. Observation of Rutherford’s experiment 4

The observations made by Rutherford were: 1. Most of the alpha particles pass through the gold foil with no deflection. 2. A few of the alpha particles deflect through the gold foil with small angles 3. A very few alpha particles deflect through the gold foil with large angles nearly 180. Failure of Thomson’s atomic model in the view of alpha particle scattering: The model cannot explain the alpha particle scattering. According to Thomson’s model, all the alpha particles should pass through the atom without suffering any deflection. Since this contradicts the experimental observations, Thomson model does not give a correct picture of atom. Conclusions of Rutherford’s experiment 1. Most of the space in an atom is empty. 2. Positive charge in an atom is not uniformly distributed and concentrated in a very small volume. 3. The volume occupied by positive charge is very small as compared to the total volume occupied by an atom. Rutherford Model of an atom: Based on the above observations and conclusions, Rutherford proposed the model of atom 5

According to the Rutherford’s atomic model, 1. The positively charged particles called protons and most of the mass was concentrated in a very small volume at the centre of the atom. This region of atom called as nucleus. 2. The negatively charged particles called electrons are revolved around the nucleus in close circular orbits called orbits. Just as in the solar system, the sun is at the centre and the plants revolve around it. 3. The atom is stable because the electrons and nucleus is held together by electrostatic force of attraction. Comparison of the Radius of Nucleus and Radius of Atom From the observation on the maximum angle of deflection of alpha particles, Rutherford measured the dimensions of the atom as: i. Radius of the nucleus is of the order of 10-15 m. ii. Radius of the atom is of the order of 10-10 m. Defects of Rutherford’s model of atom Although the Rutherford atomic model was based on experimental observations but he failed to explain the most important thing • Rutherford proposed that the electrons revolve around the nucleus in paths called orbits. According to classic law of mechanics and electrodynamics, if an electrically charged particle is in motion, it emits electromagnetic radiations. Thus, an electron revolving around the nucleus should emit electromagnetic radiation, i.e. loses energy. As a result, it should be gradually pulled towards the nucleus and would end up with the collision. This would collapse of the atom. If it was so, the atom should be highly unstable. Thus, Rutherford model failed to explain the stability of an atom. • Rutherford’s atomic model did not mention anything about the arrangement of electrons in the orbit. *** 6