Thomson stated that atom is uniform sphere of positive charge with negatively charged electron embedded in it in such a way that atom becomes electrically neutral.
A beam of α-particle was projected towards this gold foil. The foil was surrounded by circular fluorescent ZnS screen which produces flash when α-particle collide with it.
Observations of experiment:
On the basis of scattering experiment, Rutherford proposed an atomic model or nuclear atomic model or nuclear model of atoms.
Fig: Structure of atom
Radius of nucleus:
Radius of nucleus is of the order of 10-15 m and radius of atom is order of 10-10 m.
r = r0 A1/3
A = Mass number = no. of protons no. of neutrons.
r0 = constant = 1.2 × 10-13 cm.
Distance of closest approach:
If the α-particle is projected with some velocity -v from very large distance then at point (1), energy of the α-particle
As the particle is moving closer to the nucleus of other particle, repulsive coulombic force increases (P.E. increases) and velocity of the particle decreases, kinetic energy is converted into P.E.
∴ At point (2) E = K.E. + P.E. =
[As no ext. force is acting, ∴ Total energy of system remains constant].
velocity becomes zero at rmin distance where Coulombic potential energy of repulsion becomes equal to the initial kinetic energy.
At point (3),
q1 = z1e ; q2 = z2e
e = electronic charge = 1.6 × 10-19 C
z1 = z2 = Atomic number
Drawbacks of Rutherford's Model:
(1) According to Maxwell's wave theory, an accelerated charged particles (like electron) revolving in the field of another charged particle like nucleus looses energy in the form of electromagnetic radiation and if this happen then orbit of the revolving electron should keep on decreasing and ultimately electron should fall into the nucleus and atom should collapse but this doesn't happen actually.
(2) Rutherford's model didn't give any idea about electronic arrangement of an atom and since he didn't mention specific energy levels he couldn't explain the discontinuous atomic hydrogen spectrum.