An α particle is a positively charged particle.
Rutherford observed the behaviour of these particles when they approach the interior of the atoms as shown in fig. 

Rutherford observed that most of the α particles like a, a₁ , pass through the atom un deflected.

Some of the particles like b, b₁  get scattered by the atom at smaller angles of deflections. The particles like c, c₁  undergo large deflection at an angle less than 180^o  but more than 90^o . The particles like ‘d' get deflected such that they are sent back as d₁  with an angle of 180^o.
Such large - angle deflections require strong forces to be acting on the α particles. Rutherford argued that this would be possible if all the positive charges and mass of the atom was concentratedin a very small central region which he called the nucleus of the atom. Then the large angle deflections of α particles are c, c₁, due to electric repulsive forces caused by the nucleus. From the data obtained in this experiment Rutherford calculated the radius of the nucleus and found it to be shorter than 2.4×10^-15m. Because the mass of the electron is only about 1/7000th mass of an alpha particle, the effect of the presence of electrons inside the atom on the deflection of the alpha particles can safely be ignored.