Chemistry: Models Of The Atom
RUTHERFORD’S ATOMIC MODEL
Rutherford proposed the planet like model of an atom which resembles the solar system. In his model the electrons revolve around the nucleus as planets revolve around the sun. The nucleus contains protons and neutrons. As the atom is neutral on the whole this implies that an atom consists of same number of protons and electrons. Electrons are not stationary because if they have been, they would have been attracted towards the positively charged nucleus
- The main defect in this model is that it does not explain the exact behavior of electrons. And also the electron which is moving will be accelerated towards the nucleus and eventually it should fall into the nucleus.
BOHR’S ATOMIC MODEL
According to Bohr’s atomic model, electrons can exist in certain permitted quantized energy levels. The main points of this model are as under:
(a). Electrons revolve in one of the circular orbits outside the nucleus. Each orbit has a fixed energy and a particular quantum number is assigned to it.
(b). Electron revolving in a particular orbit neither emits nor absorbs energy while moving in that orbit. Energy is only emitted or absorbed when electron jumps from one orbit to another
(c). Energy is absorbed by electron when it jumps from an inner orbit to an outer orbit and is emitted when it jumps from outer to inner orbit.
(d). Electrons can revolve only in those orbits having a fixed angular momentum.
(a). This theory can explain origin of spectrum of one electron system like He+1 ,Li+2 , Be+3 but cannot explain poly electron systems like: He , Li and Be
(b). When hydrogen spectrum is viewed in high resolving power spectrometer the individual spectral lines are further formed of various fine lines. This is called fine structure or multiple structure. This means that only one quantum number cannot explain various spectral lines.
(c). Bohr proposed circular orbits but recent studies showed that electron does not revolve in a single plane but revolves in a three dimensional space.
(d). Bohr model cannot explain the Zeeman :Effect and Stark effect.
WAVE THEORY OF ATOM
Planck’s quantum theory of radiation states that light shows a dual character it acts both as a material particle and as a wave. This concept was extended to matter particles by Louis de Broglie. In Bohr’s atom, the electrons are moving with specific velocities in orbits of specific radii but according to Heisenberg’s uncertainty principle both these quantities cannot be measured experimentally at the same time. In order to solve this problem Schrodinger proposed the wave theory of atom. According to him the position of an electron cannot be found exactly, but the probability of finding an electron at a certain position at any time can be found. The volume of space in which there is 95% chance of finding an electron is called atomic orbital. The atomic orbital is different from atomic orbit of Bohr because atomic orbital can be regarded as a spread of charge surrounding the nucleus. This is often called the electron cloud.