Eric M. Alcantara
The present state of atomic theory is characterized by the fact that we not only believe that existence of atoms to be proved, but also we even believe that we have an intimate knowledge of the constituents of the individual atoms. It was in the early decades of the 19th century when the structure of the atom was coming in focus. We know that atoms consist of electrons, protons, neutrons and the nucleus. The discovery of the electron and the euclidation of its properties was the result of the work of J. J. Thompson. It was known that the protons and neutrons could group into the center region called nucleus, which was discovered by Ernest Rutherford in his gold foil experiment. But scientists could not think of any stable arrangements of particles. In 1902, Gilbert Lewis proposed a cubical model for atoms in which the electrons were positioned at the eight corners of the cube in a molecule. J. J. Thompson also suggests the plum pudding model in which electrons were jest embedded in the atom. But, both were disproved by Ernest Rutherford after his discovery of nucleus, he made a model that has a resemblance to a planetary system in which electrons orbit around the nucleus. But, Rutherford’s model violates classical electromagnetic theory because if the electrons were stationary, it would fall into the nucleus since the opposite charges on the particles would cause them to attract one another. It cannot be in an orbit circulating the nucleus either because circular motion requires consistent acceleration of circling body to keep it from flying away. Classical electromagnetic theory predicts that charged particles radiates light when it was accelerated. And this radiation will change in frequency as the electron loses energy and eventually it will spiral into the nucleus.
But by intuition, we know that the prediction is fake. Some atoms glow in the dark by emitting visible radiation but they do not change color which indicate frequency change neither they blow up or disappear.
Working under Rutherford was a brilliant young scientist who came to the rescue of Rutherford’s model. It was Neils Hendrik David Bohr who passed on to a study of the structure of the atom on the basis of Rutherford’s discovery of the nucleus. By borrowing the concept from the quantum theory of Max Planck, states that matter emitting or absorbing radiation is not continuous but in discrete bundles of energy called quantum. He suggested that the laws and behavior of the large particles are inadequate to explain all the motions and behavior of all the atoms and electrons. He proposed that an electron despite the fact that it is accelerating does not necessarily radiates energy. Bohr knew that electrons travel close to the nucleus in small orbit possess less energy than those moving in large orbits. This is because of Coulomb’s law that the electron near the nucleus is attracted more strongly by the nucleus than one that is farther away. Energy is required and work must be done in order to move it from the small orbit to a large orbit. Bohr reasoned that the energy difference between smaller and larger orbits must somehow be related to Plank’s quanta of energy. This leads Bohr to conclude that around the nucleus of an atom may occupy only certain precise orbits or energy levels, which leads him into his two postulates:
1. Every atom consists of nucleus and suitable number of electrons revolved around the nucleus in circular orbits. Electrons revolved only in certain non-radiating orbits called stationary orbits for which the total angular momentum is an integral multiple of h/2p where h is plank’s constant.
2. Radiation occurs when an electron jumps from one permitted orbit to another. It is emitted when electron jumps from higher orbit to a lower orbit.