NCERT Class 9 Science Lesson Structure of atom Solution and Notes

Structure of atom

Ionisation of gases in the discharge tube proved that atoms have subatomic particles
Summary of characteristics of electrons, protons and neutrons

Relative charge-1+10
NatureNegatively chargedPositively chargedNeutral
Discovered byJJ ThomsonE. GoldsteinChadwick

Thomson model of atom:

An atom is a uniform sphere of positive charges (due to presence of protons) as well as negative charges (due to presence of electrons). Atom as a whole is electrically neutral because the negative and positive charges are equal in magnitude

Limitations of Thomson model of atom:

Model failed to explain how protons and electrons were arranged in atom so close to each other
α particles are charged particles having two units of positive charge and 4 units of mass, i.e. they are doubly charged helium ions(He2+)
Observations predicted from Alpha- particle scattering experiment by Rutherford based on Thomson model of atom:
Rutherford expected that if the model proposed earlier by J.J Thomson, according to which there is uniform distribution of positive and negative charge, was correct then α-particle particles striking the gold atoms would be uniformly deflected which was not the case. Since the α-particles were much heavier than the protons, he did not expect to see large deflections
Selection of gold metal for Rutherford’s α – particle scattering experiment :
Since α-particles are easily absorbed by objects; he wanted to ensure that alpha particles pass through foil without getting absorbed. If α -particles were absorbed in matter then they would not be able to give any useful information about insight of atom. Only if α-particles were deflected then only it can give useful information about insight of atom. Gold is easily malleable and can be beaten into very thin sheets.

Observations made by Rutherford from α – particle scattering experiment:

Most of the α- particles passed straight through gold foil without suffering any deflection from their original path
Some of the α – particles were deflected by the foil by small angles
One out of every 12000 particles appeared to rebound

Conclusions from Rutherford’s model

Most of the space between inside atoms is empty; hence it allows the αparticles to pass straight through it without any deflection
Very few particles were deflected from their path which suggests that the positive charge of the atom occupies very little space
The total volume occupied by a nucleus is very small compared to the total volume of the atom, as very few α- particles are reflected by 180o and all the positive charge and mass of the gold atom were concentrated in a very small volume within atom.

Rutherford’s model

There is positively charged centre in an atom called the nucleus and the entire mass of atom resides in the nucleus
Electrons revolve around the nucleus in well defined orbits
Size of nucleus is very small as compared to size of atom

Defects in Rutherford model of atom:

Rutherford had proposed that electrons move around a positively charged nucleus at very high speed in circular orbits. To remain in a circular orbit electron would have to be accelerated centripetally (tending to move toward a center). But according to electromagnetic theory if charged body (electron) is accelerated around another charged body (nucleus) then there would be continuous radiation of the moving body (i.e. electron). This loss of energy would slow down speed of electron and eventually electron would fall into nucleus. But Rutherford’s model could not explain such a collapse
Rutherford had proposed that electrons revolve around the nucleus in fixed orbits. He did not specify the number of electrons in each orbit

Postulates put forward by Bohr regarding model of atom:

Electrons revolve around the nucleus in a limited number of orbits called discrete orbits of electrons or also called as permissible orbits
While revolving in discrete orbits the electrons does not radiate energy i.e. energy of an electron remains constant so long as it stays in a given orbit. Electrons present in different orbits have different energies
When an electron jumps from lower energy level to higher energy level some energy is absorbed, while energy is released when electron jumps from higher energy to lower one
Orbits or shells are represented by the letters K, L, M, N… or the numbers, n=1, 2, 3, 4…


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