According to Dalton’s atomic theory, an atom was indivisible and indestructible. Now, the discovery of two fundamental particles (electrons and protons) inside the atom, led to the failure of this aspect of Dalton’s theory. To explain the arrangement of electrons and protons within an atom, many scientists proposed various atomic models.
THOMSON’S MODEL OF AN ATOM
J.J. Thomson was the first scientist to propose a model for the structure of an atom sometimes called the ‘plum pudding model’. The electrons are placed in a sphere of positive charges, which was like currants (dry fruits) in a spherical Christmas pudding.
It can also be compared to a watermelon, in which the positive charge in an atom is spread all over like the red edible part, while the electrons are studded in the positively charged sphere, like the seeds in the watermelon.
The model was based on the following postulates:
(i) Electrons are embedded in the sphere of positive charges.
(ii) The negative and positive charges are equal in magnitude. Therefore, the atom as a whole is electrically neutral.
(iii) The mass of an atom is assumed to be uniformly distributed throughout the atom.
Limitations of Thomson’s model (Plum Pudding model) of an atom are:
(i) It could not explain the experimental results of other scientists such as Rutherford, as there was no nucleus in the atomic model proposed by Thomson.
(ii) I could not explain the stability of an atom, i.e. how positive and negative charges could remain, so close together.
RUTHERFORD’S MODEL OF AN ATOM
Ernest Rutherford designed an experiment to know how the electrons are arranged within an atom. It is known as the ‘Rutherford’s gold foil experiment’. He bombarded fast moving a-particles (alpha particles – doubly-charged helium ions having a mass of 4 u) on a thin sheet of gold foil.
(i) Since a-particles have a mass of 4 u, the fast-moving α-particles have a considerable amount of energy.
(ii) He selected a gold foil because he wanted a layer as thin as possible. This gold foil was about 1000 atoms thick.
(iii) Since the α-particles were much heavier than the protons, he did not expect to see large deflections.
The following observations were made by Rutherford during his experiment:
(i) Most of the fast-moving a-particles passed straight through the gold foil.
(ii) Some of the a-particles were deflected by the foil by small angles.
(iii) Very few α-particles (one out of 12000) appeared to rebound.
On the basis of his experiment, Rutherford concluded that:
(i) Most of the space inside the atom is empty because most of the α-particles passed through the gold foil without getting deflected.
(ii) Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.
(iii) A very small fraction of α-particles were deflected by 180° (i.e. they rebound), indicating that all the positive charge and mass of atom were concentrated in a very small volume within the atom.
On the basis of his experiment, Rutherford put forward the ” nuclear, model of an atom”, having the following features:
(i) There is a positively charged, highly dense centre in an atom, called nucleus. Nearly, the whole mass of the atom resides in the nucleus.
(ii) The electrons revolve around the nucleus in a circular path.
(iii) The size of the nucleus (10-15 m) is very small as compared to the size of the atom (10-10m).
Note Rutherford suggested that his model of atom was similar to that of solar system. In the solar system the different planets are revolving around the Sun. Similarly, in an atom the electrons are revolving around the nucleus. So, these electrons are also called planetary electrons.
Limitations of Rutherford’s Model of an Atom
(i) Any charged particle when accelerated is expected to radiate energy. To remain in a circular orbit, the electron would need to undergo acceleration. Therefore, it would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus. If this were so, the atom should be highly unstable, which is not the case as atoms are stable. Thus, it could not explain the stability of an atom.
(ii) Rutherford’s model could not explain the distribution of electrons in the extra nuclear portion of the atom.
BOHR’S MODEL OF AN ATOM
To overcome the objections raised against Rutherford’s model of the atom, Neils Bohr put forward the following postulates about the model of an atom:
(i) Only certain special orbits known as discrete orbits of electrons are allowed inside the atom.
(ii) While revolving in discrete orbits the electrons do not radiate energy.
(iii) Each of these orbits is associated with a certain value of energy. Hence, these orbits are called energy shells or energy levels. As the energy of an orbit is fixed (stationary), an orbit is also called a stationary state.
(iv) Starting from nucleus, energy levels (orbits) are represented by numbers (1, 2, 3, 4 etc.) or by alphabets (K, L, M, N etc.).
(v) The electrons present in first energy level (E1) have the lowest energy. Energies increases on moving towards outer energy levels.
(vi) Energy of an electron remains same as long as it remains in discrete orbit and it does not radiate energy while revolving.
(vii) When energy is supplied to an electron, it can go to higher energy levels. While an electron falls to a lower energy level when it radiates energy.