Class 9th – STRUCTURE OF ATOM (Models)

DIFFERENT MODEL OF ATOM PROPOSED BY DIFFERENT SCIENTIST

MODEL PROPOSED BY J.J THOMSON

According to him, the atom consist of positively charged sphere and all the electrons are embedded in it. The positive and the negative charge are equal in number, thus making the atom neutral .

POSTULATES:

Atom consist of positive charge all over the atom and the negatively charged electrons are embedded in it. ( as shown in the figure)
Number of positive and negative charge are equal in number. i.e. the magnitude of both the charges are equal.

DRAWBACKS:

It failed to explain how is an atom stable.
How does the positive and negative charge hold each other.
Did not explained anything about the nucleus.

RUTHERFORD’S MODEL OF AN ATOM

Rutherford performed an experiment, where the α particles and gold foils were used.
The α particles are positively charged particle which consist of 2 unit of positive charge and the mass is 4 unit.
The α particles is He²⁺ ion.
These α particles are emitted by different radioactive elements like polonium.
The α particles have the tendency to penetrate through matter to some extent.
He bombarded the α particles to the gold foil. When the α particles were passed through the gold foil the following observations were observed.

OBSERVATION

Most of the α particles was seen that they passed straight through the gold foil, that is there was no deflection in the path of the α particles.
Few α particles deflected through different angles.
Few α particles rebounded back in the same path after hitting the gold foil.

RESULT

As most of the α particles pass straight through the foil, this shows that there is lot of empty space inside the atom.
As few α particles deflected through some angles, this was because as the same charges repel each other. Since the α particles have positive charge and the region where these particles got deflected must surely have the same charge.
As few rebounded back this was due to the nature at the center. the center was dense and hard and this resulted in rebounding of the α particles.

CONCLUSION

The empty space shows that the positive charge is localized at a particular place inside the atom.
Since few α particles got deflected, so that particular part consist of the NUCLEUS. Thus nucleus of an atom consist of positive charge.
The nucleus of atom is small and dense.
Thus, the atom consist of positive charge dense nucleus at the center. The nucleus is surrounded by the negative charge and most of the space inside the atom is empty.

DRAWBACK

He stated that the negatively charged electrons are revolving around the nucleus( positive charge) in a circular pathway. The object that moves in a circular pathway have accelerated motion, thus electrons have accelerated motion.
As charged particle undergo accelerated motion, they will radiate energy and thus the electrons while moving will radiate energy and point will come when their will be loss of energy of the electron and soon it will be attracted by the nucleus which consist of the positive charge.
The electron will finally fall into the nucleus and will result in the atom to be unstable and will result in the collapsing of the atom.
As we know that atoms are very stable and they cannot collapse on their own, so Rutherford was unable to explain the stability of the ATOM.

BOHR’S MODEL OF AN ATOM

Neil Bohr was able to explain the stability of atom as he stated that the electrons revolve around the nucleus in orbits, which have different radius.
The electrons which are near to the nucleus have low energy and the electrons which are far from the nucleus have high energy.
Since, when the electrons revolve around the nucleus they do not radiate energy and thus the atom remains stable.
The circular path can also be known as Electron shells.

BOHR’S MODEL

The atom consist of proton, neutron and electrons. The proton and neutron are present inside the nucleus.
The electrons are revolving around a fixed orbit called as electron shells. The shells are named as K,L,M,N………
K shell can hold maximum of 2 electrons in it.
L shell can hold maximum of 8 electrons in it.

ARRANGEMENT OF ELECTRONS IN THE ELECTRONIC SHELL

K SHELL= 2 electrons
L SHELL= 8 electrons
M SHELL= 18 electrons.
The outermost shell have the highest electrons in it.
To know the number of electrons present in a shell, we can find it through the formula, No. of electrons = 2n²
The outermost shell of an atom cannot accommodate more than 8 electrons even if they have the capacity to hold electrons, except K shell.
Electrons do not occupy the next shell until the 1st shell completely fills up by the required no. of electrons.
For example, the atoms of oxygen has atomic no.= 8, then the electron present at K shell= 2 and L shell= 6 electron. Thus K + L = 2+6= 8 Electron.