In 1911, Rutherford performed alpha rays scattering experiment to demonstrate and to find the structure of atom.
Heavy atoms have a heavy nucleus carrying a great quantity of positive charge. Hence, some alpha particles are easily deflected back on hitting the nucleus. Besides a bit of alpha particles are deflected through small angles because of large positive charge on the nucleus. So with the aid of heavy nuclei atoms it was possible to understand or detect the minute deflections of alpha particles.
If light atoms are used, their nuclei will be light & moreover, they will hold a small positive charge on the nucleus. Hence, the number of particles deflected back & those deflected through some angle will be trifling. Hence deflections of alpha particles from light atoms remain unnoticeable.
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In Rutherford’s eNCERT - Chemistry Part-I