Modern atomic theory provides a reasonably satisfactory explanation of the properties of matter, the mechanisms of chemical change and the interaction of matter and energy. Such a theory emerged from the synthesis of the work of several scientists, only a few of which will be discussed here.
John Dalton (1766-1844) is generally credited as the father of the atomic theory but the early Greek philosophers were the originators of the concept of atoms. They taught that matter is composed of atom and is therefore finitely divisible, John Dalton considered the atom as constituting the simplest component of matter. He viewed the atom as ‘indestructible’, tiny hard spheres. The discovery of radioactivity by Henry Becquerel (1891 showed that atoms are complex rather than ‘indivisible’ or ‘indestructible’ but can disintegrate forming atom of different elements. The discovery of cathode rays in electric discharge tubes (1895) by William Crookes revealed that negatively charged electrons were components of the atom.
By 1900, it was already established that matter consists of atom but nothing was known about the structure of the atom. It was only known that the atoms contain electrons but that on the whole the atom was electrically neutral. This neutrality means that there must exist within the atom enough positively charged components to balance the negatively charged electrons. This led Sir J. J. Thomson the English Physicist, to propose an atomic model which visualized the atom as a homogeneous sphere of positive charge inside of which are embedded negatively charged electrons as shown below
- J. Thomson also determined the ratio of the charge to mass, e/m, of electrons, and found e/m to be identical for all cathode-ray particles, irrespective of the kind of gas the in the tube or the metal the electrodes are made of.
Rutherford Model – the Nuclear Idea and the Planetary Model
Ernest Rutherford (1911) and his co-workers performed experiments a beam of positively charged alpha (α) particles was directed at a pin sheet of a metal foil. It was found that most of the alpha-particles passed through the foil without deflection as if the foil were mostly empty space. Only a few of them were diverted from their paths. Some of these few actually rebounded backwards.
The scattering of alpha-particle by the metal foil was explained as a repulsion from a heavy positively charged nucleus present at the centre of the atom of the metal foil. This follows because an abrupt change in path (as noted for a few α-particles) of a relatively heavy and positively charged α-particle can result only from its hitting or from its close approach to another particle (the nucleus) with a highly concentrated, positive charge. All these contradicted Thomson’s atomic model which supposed that the distribution of the charges was diffuse. Hence Rutherford proposed his model of the atom.
Rutherford proposed a planetary model of the atom which suggested that the atom consists of a positively charged heavy core called the nucleus which most of the mass of the atom was concentrated. Around this nucleus negatively charged electron circle in orbits much as planets move around the sun. Each nucleus must be surrounded by a number of electrons necessary to produce an electrically neutral atom.
This model was a major step toward how we view the atom today. It however had two problems. According to Newtonian Physics, such an atom as Rutherford’s would collapse by spiraling into the nucleus, since there is an attractive force between the oppositely charged nucleus and electrons. Further experiments indicated that charged moving in a field of opposite charge lose energy by emitting radiation. But atoms in their normal state neither collapsed nor emitted radiation.
Thus the two main difficulties of the Rutherford model are these:
- It predicts that light of a continuous range of frequencies will be emitted, whereas experiment shows line spectra instead of continuous spectra.
- It predicts that atoms are unstable – electrons quickly spiral into the nucleus – but we know that atoms in general are stable, since the matter around is stable.
Clearly Rutherford’s model was not sufficient to explain experimental observations. Some sort of modification was needed and this was provided by Neils Bohr.