compare the models of rutherford, bohr and thomson

• rutherford's model of atom

The major features of the Rutherford atomic model are as follows.

1. All protons are present inside the nucleus, which is situated at the centre of the atom.
2. Electrons reside outside the nucleus and revolve around the nucleus in well-defined orbits.
3. The size of the nucleus is very small in comparison to the size of an atom. As per Rutherford’s calculations, the size of the nucleus is 105 times smaller than an atom.
4. As the mass of the electron is negligible in comparison to the mass of the proton, almost all the mass of the atom is concentrated in the nucleus.

2. Drawbacks of Rutherford’s model are listed below:

• Rutherford’s model failed to explain stability of atoms. According to Rutherford’s model, electrons are revolving around the nucleus. This means, electrons would be in a state of acceleration all the time. Since electrons are charged particles, therefore, electron revolving in a circular orbit should continuously emit energy. It is because whenever a charged particle is accelerated it emits energy. As a result of this, revolving electron would lose energy and would slow down. So, it would no longer be able to resist the attractive force of the nucleus. Hence, it would move closer and closer to the nucleus and would finally fall into the nucleus by following a circular path. This means atom should collapse but actually we know atom is stable.
• Rutherford’s model does not explain the distribution of electrons around the nucleus.
• Rutherford’s model does not tell anything about the energy of electrons

• bohr's model

Neils Bohr gave atomic model in 1913.The main points of this model are:

1) An atom consists of a central positive part called the nucleus which contains all the protons and neutrons.

2) Electrons which are present outside the nucleus revolve around the nucleus in definite circular paths called orbits or shells.

3) Each orbit has a fixed energy and the orbitals are represented by the symbols K, L,M, N etc or by the numbers 1,2,3 …

4) Each orbit can accommodate not more than a fixed number of electrons.

5) Electrons moving in the same orbit donot lose or gain energy.

6) Electrons lose energy on jumping from a higher orbit to lower one  and vice-versa.

• thomson's model

• According to this model, an atom can be considered as a sphere of uniformly distributed positive charge in which there are electrons distributed symmetrically.

• The electrons must be held by the positive charges by electrostatic forces. The mutual repulsions between the electrons are balanced by the force of attraction towards the centre of the sphere.

• In an atom with a single electron like the hydrogen atom, the electron must be situated at the centre of the positive sphere.

• In an atom with two electrons, like the helium atom, the two electrons must be symmetrically situated on opposite sides of the centre at a distance equal to half the radius of the positive sphere.

• In the three-electron system, the electrons should be at the corners of a symmetrically placed equilateral triangle, the side of which is equal to the radius of the sphere.

• Proceeding in this manner, Thomson could explain the arrangement of electrons ranging from 1 to 100 inside the positive sphere.

• This model was also called 'the plum pudding model '. The electrons are like plums in a pudding (positive charge). The seeds in a watermelon fruit can be given as a parallel comparison.

• With this model he tried to explain the observed spectra of elements.

• According to him, the vibrations of electrons would give rise to electromagnetic energy radiated. He could explain with some success the process like chemical reaction, radioactivity etc.

Drawback of the Ruthorford 's model:

• The model is applicable for one electron system.

• According to Rutherford as the electron revolves around the nucleus it liberate energy regularly so when the electron loses all its energy it should fall in the nucleus which was impossible.

• The model does not give the objection raised by Clerk Maxwell related to continuous emission of energy by electron.

• The Rutherford’s model of atom does not say anything about the arrangement of electrons in an atom.

Isotope:

• Isotopes are defined as atoms having the same atomic number, but different mass numbers. These atoms contain an equal number of protons and electrons, but a different number of neutrons.

For example, in nature, hydrogen is found in three forms with different mass numbers, namely protium (), deuterium(), and tritium (). These are the three naturally occurring isotopes of hydrogen. The atomic number of each isotope is 1, but the mass number varies i.e. it is 1, 2, and 3 respectively.

thubs up plzz !