`=>` J.J.Thomson proposed that atoms can be considered as a large positively charged body with a number of small negatively charged electrons scattered throughout it. This model was called as Plum pudding model of the atom.

`=>` The electrons represent the plums in the pudding made of positive charge. It is sometimes also called as Watermelon model. In this, the juicy pulp of the watermelon represents the positive charge and the seeds represent the electrons.

`=>` Thomson model was discarded because it could not explain certain experimental observations like alpha particle scattering experiment by thin metal foils conducted by Ernest Rutherford.

In 1909, Rutherford discovered proton in his famous gold foil experiment. In this experiment, Rutherford bombarded a beam of alpha particles on an ultrathin gold foil and then detected the scattered alpha particles in zinc sulfide (`ZnS`) screen.

Results : `=>` Most of the particles pass through the foil without any deflection.

`=>` Some of the alpha particles deflect at small angle.

`=>` Very few even bounce back (`1` in `20,000`).

Conclusion : Based on his observations, Rutherford proposed the following structural feature of an atom :

`=>` Most of the atom's mass and its entire positive charge are confined in a small core, called nucleus. The positively charged particle is called proton.
`=>` Most of the volume of an atom is empty space.
`=>` The number of negatively charged electrons dispersed outside the nucleus is same as number of positively charge in the nucleus. It explains the overall electrical neutrality of an atom.
`=>` The negatively charged electrons revolving around the nucleus are called planetary electrons.
`=>` The size of the nucleus ( `r = 10^(-15) m`) is very small as compared to that of atom (`r = 10^(-10) m`).

Drawback of the Rutherford's model :

`=>` A charged particle undergoing acceleration would continuously emit radiation and lose energy. Since the electron in the atom is also
a charged particle and is under acceleration, it is expected to continuously lose energy. As a consequence, the electron moving around the nucleus would approach the nucleus by a spiral path and the atom would collapse.

`=>` Since above does not happen it can be said that the Rutherford's model failed to explain the stability of the atom.

`text(Note) :` During `alpha`-particle scattering experiment, `alpha` - particles were subjected to bombard a thin sheet of heavy metals like gold, platinum because their nuclei is large thus provides good results.

In order to account for the stability of the atom, Neils Bohr-a Danish physicist, modified Rutherford's model and put forward his theory in 1913.The assumptions of Bohr's Theory are as follows:

`=>` Electrons revolve round the nucleus in definite orbits called stationary states.
`=>` Each stationary state is associated with a definite energy, which is called an energy level.
`=>` As long as electrons revolve in the stationary states, they don't lose or gain energy.
`=>` Electrons may jump from one orbit to another, in which case energy is absorbed or emitted in fixed quantities only (known as 'quanta').
`=>` Only those orbits are permitted in which the angular momentum (`mvr`) of the electron is a whole number multiple `h/(2pi)` (`h` is a Plank's constant) i. e. `mvr = ( nh)/(2pi)` where `n = 1 , 2 , 3 ...........`

`=>` The frequency of radiation absorbed or emitted when transition occurs between two different energy levels or states is given by

`nu = ( Delta E)/h = (E_2 - E_1)/h`

where, `E_1` and `E_2` are the energies of lower and higher energy states.

`=>` The energy of an electron residing in a particular energy level (`n`) is given by

`E_n = -21.8xx10^(-12) xx Z^2/n^2 erg at om^(-1)`

` = -21.8xx10^(-19) xx Z^2/n^2 J at om^(-1)`

` = -13.6xx z^2/n^2 eV at om^(-1)`

`E = ( h c)/lamda_n = - R_H (1/n_2^2 - 1/n_1^2) Z^2`

`=>` Radius of nth orbit `(r_n) = (0.53 n^2)/Z A^0`
`=>` VeIocity of electron in nth orbit ` = (2.08xx10^8 Z)/n cm//s`

`=>` Various experiments showed that small particles such as electrons behave both as particles (solid objects) and as waves (Wave-particle duality). Because of this it is not possible to precisely locate the electron and measure its velocity simultaneously. Hence there is uncertainty associated with both these quantities.

`=>` In Bohr's model, electrons are assigned orbits or fixed trajectories, which is contrary to the principle above. Hence even Bohr's model failed to explain atomic structures completely.

`=>` He could not explain the splitting of spectral lines in the presence of magnetic field (Zeeman effect) and electric field (Stark effect).
`=>` He could not explain the line spectra of atoms containing more than one electron called multi-electron atoms.
`=>` He could not explain the three dimensional existence of atom.

Rydberg gave a very simple theoretical equation for the circulation of a wavelength of number of lines present in hydrogen spectrum `bar (v) = R (1/(n_1^2) - 1/(n_2^2))` When electron jumps from `n=2` to `n=1` the hydrogen atom emits radiation of the shortest wavelength.

Energy level	Energy (Joule atom `text()^(-1))`
n = 1	`-21.79xx10^(-19)`
n = 2	`-5.42xx10^(-19)`
n = 3	`-2.41xx10^(-19)`
n =4	`-1.36xx10^(-19)`
n = 5	`-0.87xx10^(-19)`
n = `oo`	0

The largest absorption of energy will be for transition `n = 1` to `n = 2`. For transition `n = oo` to `n = 1`, is although maximum but in this transition energy will be released.