Rutherford performed an experiment by bombarding a-particles (Helium nuclei Z = 2, A = 4 on a gold foil.
(i) Most of α-particles pass through the gold foil undeflected.
(ii) A very small number of α-particles (1 in 8000) suffered large angle deflection; some of them retraced their path or suffered 180° deflection.
(i) Atom is hollow.
(ii) Entire positive charge and nearly whole mass of atom is concentrated in a small centre called nucleus of atom.
(iii) Coulomb’s law holds good for atomic distances.
(iv) Negatively charged electrons are outside the nucleus.
Impact Parameter : The perpendicular distance of initial velocity vector of α-particles from the nucleus, when the particle is far away from the nucleus, is called the impact parameter. It is denoted by b. For head on approach of a-particle, b = 0.
Angle of Scattering : The angle by which -particle is deviated from its original direction is called angle of scattering.
where EK is the initial kinetic energy for head on approach of alpha particle.
Impact parameter, b =0.