Scattering of alpha Particles :-
α-particles are doubly charged Helium nucleus. Thus a mass of α-particle is four times the mass of hydrogen and contains positive charge two times the charge of a proton.
The α-particles are continuously shot(emitted) from a radioactive substance like radium and usually carry high energy.
If a sharply defined α-ray beam is incident on thin gold foil of thickness approx. 0.0001 m beyond which a photographic plate is placed.
The two particles are scattered from the gold foil & following observation are seen :-
i) Most of α-particles passes straight through the gold foil indicating that most of part of atom is hollow.
ii) Some of α-particles were scattered through a small angle but few of them were deviated through large angle.
Explanation of α-particle Scattering :-
Rutherford assumed that when an α-particle(charge =2e) approaches the positive nucleus (charge=Ze), Z is atomic number. It experiences a Coulomb repulsion force.The Coulomb repulsive force is given by :
Due to this electrostatic repulsive force, the α-particles get deflected.
If ‘r’ is large, i.e. α-particle is passing away from the nucleus. It will experience less force and hence is deflected through small angles.
Rutherford’s Model of Atom:-
On the basis of scattering of α-particles by heavy nuclei, Rutherford in 1911 proposed a new atomic model known as “nuclear atomic model”.
According to it, the atom consists of central heavy nucleus of size 10^-15 m in which entire positive charge and almost the entire mass of atom is concentrated. The nucleus is surrounded by cloud of electrons whos3 total negative charge is equal to positive so that atom as a whole is electrically neutral.
The size of atom is in order of ~10^-15 m.
Those electron cannot be at rest because it is so they will be attracted by positively charged nucleus and will fall into it.
Rutherford therefore considered that these electrons are revolving around the nucleus in circular orbit, like the planet move around the sun.
Drawbacks of Rutherford’s Atomic Model :-
i) According to electromagnetic theory, a revolving electron should radiate continuously. If it is so, the energy of electron will go on decreasing, resulting in continuous decrease in radius of circular path and as a result the electron will approach towards the nucleus, which make atom unstable which is in opposition to Rutherford’s model.
ii) According to Rutherford model electrons un any orbit of any radius and hence they must emit radiations of all frequencies, this is not an agreement with experimental observations of atom like hydrogen which emits line spectrum of fix frequency only & not of frequencies. This is another drawback of this model.