RBSE Class 12 Physics Notes Chapter 12 Atoms

These comprehensive RBSE Class 12 Physics Notes Chapter 12 Atoms will give a brief overview of all the concepts.

RBSE Class 12 Physics Chapter 12 Notes Atoms

Structure of Atom:
According to J.J. Thomson atom is a sphere of positive charge having radius of the order of 10~ 0 m in which positive charge is uniformly distributed over the entire sphere and electrons (negative charged particles) are embedded in it like plums in pudding, called plum pudding model. This model could not explain

• origin of spectral lines of hydrogen and hydrogen like atoms.
• large angle scattering of a-particles from thin foils.

Rutherford scattering experiment
Rutherford bombarded a-particles on thin gold foil (10^-7 m thick) and found that

• Most of a-particles scattered through small angles
• A few of a-particles were reflected nearly straight back.

From this experiment he concluded that

• major portion of atom is empty space
• entire positive charge of the atom is concentrated in small central core of the atom called nucleus.

Impact parameter (b):
Impact parameter (b) is the perpendicular distance of the velocity vector of α - particle from the centre of the nucleus, when it'is far away from the atom.
And b is given by
b = \(\frac{1}{4 \pi \varepsilon_0} \frac{\mathrm{Ze}^2 \cot \frac{\theta}{2}}{\mathrm{E}_\alpha}\)

Distance of closest approach (r₀) is minimum distance upto which an α-particle travelling directly towards the centre of the nucleus can move before coming to rest and then retrace its path.
And r₀ is given by
r₀ = \(\frac{1}{4 \pi \varepsilon_0} \frac{4(\mathrm{Z} e)(2 e)}{m u^2}=\frac{1}{4 \pi \varepsilon_0} \frac{4 \mathrm{Z} e^2}{m u^2}\)

Drawbacks of Rutherford model
This model could not explain

• The stability of the atom
• Why the atom emits line spectrum?