Atoms and the Bohr Model

 INTRODUCTION

The concept of the atom was introduced by John Dalton in 1803 to explain the

chemical combination of elements to form compounds. The idea got further

confirmation when kinetic theory was developed to explain the behaviour of

gases. However, real understanding of the structure of the atom became possible

after the discovery of the electron by J.J. Thomson in 1897 and the realization

that all atoms contain electrons. The electron is a negatively-charged particle

having mass which is very small compared to the mass of an atom. Therefore,

the atom must also contain positively-charged matter, having mass almost equal

to the mass of the whole atom. Thomson suggested the plum-pudding model

of the atom, according to which the electrons are embedded in a uniform sphere

of positively-charged matter so that the atom as a whole is neutral.

Alpha-Scattering Experiment

In order to test the Thomson model, Geiger and Marsden carried out the following

experiment in 1908 under the guidance of Rutherford. Alpha-particles from a radioactive source were collimated into a narrow beam and then allowed to fall

on thin metal foils. The a-particles scattered in different directions were

detected. It was found that (a) most of the a-particles passed through the gold

foil without appreciable deflection, and (b) some of the a-particles suffered

fairly large deflections—in fact an unexpectedly large number even retraced

their path.

The Rutherford Nuclear Model

It is obvious that the above results cannot be explained on the basis of

Thomson’s model. Observation (a) requires that most of the space in the metal

foil must be empty. Observation (b) requires that the positively-charged matter

in an atom cannot be uniformly distributed but must be concentrated in a small

volume. Based on these facts, Rutherford proposed a new model known as the

nuclear model or the planetary model. According to this model, the whole of

the positive charge, which carries almost the entire mass of the atom, is concentrated

in a tiny central core called the nucleus. The electrons revolve around

the nucleus in orbits, leaving most of the volume of the atom unoccupied.

Difficulties with the Rutherford Model

The nuclear atom proposed by Rutherford could not be accepted due to the

following problems. An electron moving in a circle is continuously accelerated

towards the nucleus. According to classical electromagnetic theory, an

accelerated charge radiates electromagnetic energy. As such, the energy of the

electron would continuously decrease, its orbit would become smaller and

smaller and ultimately it would spiral into the nucleus. However, we know that

this does not happen and atoms are stable. Further, according to the classical

theory, the frequency of the radiation emitted by the electron is equal to the

frequency of revolution. Therefore, the spiralling electron would emit radiation

of continuously increasing frequency till it falls into the nucleus. However,

atoms do not radiate unless excited, in which case they radiate discrete, rather

than continuous, frequencies. We discuss this in more detail below.