Question

Compare the 3 atomic models proposed by Thomson, Rutherford & Bohr. [Class 9]

Answer 1

Thomson's model of an atom:
1. An atom consists of a sphere of positive charge with negatively charged electrons embedded in it.
2. The positive and the negative charges in an atom are equal in magnitude due to which an atom is electrically neutral.

Rutherford's model of an atom:
1. There is a positively charged, highly densed centre in an atom called the nucleus. Nearly the whole mass of an atom resides in the nucleus.
2.The electrons revolve around the nucleus in circular path.
3. The size of the nucleus is very small compared to the size of an atom.

Bohr's model of an atom:
1.An atom is made up of 3 particles electrons, protons and neutrons. Due to the presence of equal number of electrons and protons, atom on the whole is electrically neutral.
2. The protons and neutrons are located inside the nucleus due to which nucleus is positively charged.
3. The electrons revolve around the nucleus in fixed paths called orbots or shells or energy levels. The energy levels are represented as K,L,M,N or 1,2,3,4,5,.....
4. There is a limit to the number of electrons each energy level can hold. It is given by the formula 2n^2 where n is the orbit number .
5. Each energy level is associated with fixed amount of energy, the shell nearest to the nucleus having minimum amount of energy and the one farthest having maximum energy.
6. There is no change in the energy of electrons as they keeping revolving in the same shell and the atom remains stable.

Hope it helps...

Answer 2

THOMSON'S MODEL OF ATOM

Thomson's model of atom was similar to a Christmas pudding. The electrons are scattered like currants in a sphere of positive charge that is the spherical Christmas pudding.

Watermelon is also a good example of the model of atom.Here positive charge is spread like the red part of watermelon while electrons are like the seeds that are studded to it.

Postulates :
1) Electrons are embedded in the sphere of positive charge.
2) The negative charge and positive charge cancel each other. Hence, the atom is neutral.
3) Mass of atom is due to electrons.

Limitations :
1)But Thomson's model could not explain the observations made by other scientists like Rutherford's who found out that there is a nucleus in a atom but Thomson's model didn't have any nucleus.
2) It didn't have any experimental support, Thomson just proposed the model.

RUTHERFORD'S MODEL OF ATOM

Rutherford's model of atom was similar to the solar system.The planets revolve around the Sun. Similarly, the electrons revolve around the nucleus. He called these electrons planetary electrons.

Postulates :
1) There is a positively charged,highly densed center in an atom, called nucleus.Almost, the whole mass of atom resides in the nucleus.
2) The electrons revolve around the nucleus in an arbitary manner.
3) The size of a nucleus is very small as compared to atom.A nucleus can be compared with a cricket ball in a cricket stadium which is the atom.

Limitations :
1) Any charged particle is expected to radiate energy when accelerated.To remain in the orbit the electron has to undergo acceleration,so it would radiate energy.
The electron after radiating energy will finally collapse into the nucleus.If this happens then matter would not exist;but matter exists so atoms are stable.

BOHR'S MODEL OF ATOM

Bohr's model of atom is the accepted model of atom. It overcomes all the limitations of other models of atom.

Postulates :
1) Atoms have positively charged nucleus, around which electrons revolve in discrete orbits, that is electrons revolve in certain defined orbits not just any orbit.
2) Each orbit is associated with some energy. These are also called energy shells or energy levels.The energy of each orbit is fixed ,hence, they are stationery.
3) The energy level starts from nucleus.It starts from K,L,M,N....or from numbers 1,2,3,4....
4) The electrons present in first energy shell has lowest energy, energy increases when going to outer energy levels.
5)Energy of an electron remains as long as it remains in discrete orbit and it doesn't radiate energy when they revolve.
6)The electrons when supplied with energy jump to higher energy levels,when it radiates energy it falls to lower energy shell.