Rutherford’s Atomic Model

 

Ø    Rutherford (1911) did an experiment in which bombardment of thin gold coated with  ZnS with high speed positively charged α – particles emitted from radioactive substance. This experiment leads to discovery of nucleus. 

   In this experiment, a piece of radioactive substance (Radium) is placed in a lead block.Radium emitting a beam of alpha particle in all directions. The block is constructed in such a way with slits that only a narrow beam of α – particle could escape. Thin stream of α – particles comes out of holes and remaining absorbed by lead plate. This beam of α – particle then passes through thin gold foil. In order to detect α – particle after scattering, a movable circular screen coated with Zinc Sulphide is placed around the gold foil. When α – particles hit the screen, they produce flash of light which could be counted. 

                                             Figure: Rutherford’s α-rays scattering experiment

Ø Result of this experiment 

·        Most of α – particle (99%) passed through foil without undergoing any deflection

·        Few α – particles underwent deflection through small angel

·        Very very few (only one of 10000) were deflected back

Explanation

 

To deflect an α – particle right back to its path, somethings of its own mass and charge is needed. The electron are too small as compared tp α – particle ( an α – particle is 7000 times heavier than an electron) to deflect α – particle out of their course.

·        Since some α – particle were deflected back and α – particles are heavy paericle, these would be deflected back only when they strike some heavier body(nucleus) inside the atom most of α – particle passed through the foil means that remaining part of atom is hollow. So, Rutherford conclude that nucleus of atom contains most of atomic mass.

·        Since few α – particle were deflected through small angle and α – particle were positively charged particle, these could be deflected only by some positively body present within the atom (nucleus) i.e. nucleus of atom is positively charged.

·        Since most of α – particle passed through the foil without undergoing any deflection, there must be sufficient empty space within the atom i.e. atom consist of mostly empty space.

Postulates:

·        Atom has a tiny dense central core or nucleus which contains the entire mass of atom.

·        Atom consists of mostly empty space.

·        The size nucleus is extremely small as compared to size of atom.

·        Nucleus of atom is positively charged.

·        As the atom on the whole is neutral, there is one electron for each positive charge on nucleus.

·        The atom consists of central nucleus surrounded by electron which is revolving around the nucleus like planet revolving round the sun.

·        Electron and nucleus are held together by coulombic force of attraction.
Note:
Radius of atom is 10^-8cm which is 100000 time greater than the radius of nucleus (radius of nucleus = 10^-13cm)

Drawback of Rutherford’s Atomic Model

·        When an electron revolves around the nucleus, it will radiate out energy, resulting in the loss of energy. This loss of energy will make the electron to move slowly and consequently it will be moving in a spiral path and ultimately falls on the nucleus. Thus, such an atomic model and atom are unstable. Fortunately, the atom is stable.

·        Atomic spectra of the elements should be continuous band spectra but atoms are characterized by line spectra (discontinuous)