Rutherford's scattering experiment:
The concept of j.j thomson was, however,proved to be wrong by the classic scattering experiments of lord rutherford.In his experiments,a narrow beam of high speed a-particles (alpha particles are positively charged particles with two units of positive charge and mass nearly four times that of a proton.In other words, these particles are nothing but the he nuclei)emitted from a radioactive element radium were directed towards a thin sheet of gold (nearly 100 nm thick).A movable screen coated with zinc sulphide was placed at the back of gold sheet to detect whether alpha particles undergo any deviation in their path on passing through the sheet or not.
It was observed that:
(1).Most of the alpha particles passed straight through the gold foil ( about 99%0.
(2). Some of these were deflected by small angles.
(3).A very few alpha particles (only one in 20,000) suffered major deflections by more than 90 degree and even came back in the same direction.
On the basis of these observations,rutherford drew the following conclusions regarding the structure of atom:
(1).Most of the atom is hollow.
(2).The central portion of the atom carries positive charge which repels the positively charged alpha particles.
(3).The central portion of atom is solid and carries nearly the whole of the mass of the atom.
Rutherford's experiment of bohr's model:
The main features of his model are given below:
(1).An atom consists of a small but massive positively charged portion at its centre, called the nucleus. The whole mass of the atom is associated with nucleus.
(1).An atom consists of a small but massive positively charged portion at its centre, called the nucleus. The whole mass of the atom is associated with nucleus.
(2).The nucleus of each atom is surrounded by extremely small negatively charged particles called electrons.
(3).The number of electrons around the nucleus is equal to the net positive charge on the nucleus.
Rutherford's model of atom had two main drawbacks as given below:
(1) Instability of the atomic structure: Niel's bohr pointed out that rutherford's model of the atom should be highly unstable since it postulates that electrons are moving under the attractive force of the positively charged nucleus.
Rutherford's model of atom had two main drawbacks as given below:
(1) Instability of the atomic structure: Niel's bohr pointed out that rutherford's model of the atom should be highly unstable since it postulates that electrons are moving under the attractive force of the positively charged nucleus.
He argued that the motion of electrons around the nucleus would be governed by electromagnetic theory proposed by clark maxwell. According to this theory,electrons moving under the influence of attractive field of nucleus would continuously lose energy in the from of electromagnetic radiations. This loss of energy would slow down the speed of electrons.Consequently,the orbit would become smaller and smaller.Hence the electron will start revolving around the nucleus in a descending spiral and ultimately the electron would fall down into the nucleus.
(2) Failure to explain spectra: Rutherford's model of atom could not explain the formation of line spectrum of hydrogen.According to rutherford's model of atom,the spectrum of hydrogen should have been continuous and not consisting of well defined lines.