Notes : Bohr's Model of Hydrogen Atom: Postulates , FAQ and Quiz - Class 12 Physics
Learn Bohr's Theory of Hydrogen Atom in a simple and exam-oriented way. This Class 12 Physics article covers the limitations of Rutherford's atomic model, Bohr's three postulates, stationary orbits, quantization of angular momentum, and the emission and absorption of radiation. The notes are based on NCERT Physics and include FAQs and an interactive MCQ quiz with answers to help students revise effectively for board examinations and competitive exams
Why was Rutherford's Model Unsuccessful?
1. Stability of the Atom
According to classical electromagnetic theory:
- An electron moving in a circular orbit is an accelerated charged particle.
- Accelerated charged particles continuously emit electromagnetic radiation.
- As the electron emits radiation, it loses energy.
- Consequently, the radius of its orbit decreases continuously.
- The electron should therefore spiral into the nucleus.
If this happened, atoms would collapse and could not exist in stable form. However, atoms are known to be stable. Thus, Rutherford's model failed to explain atomic stability.
2. Hydrogen Spectrum
If electrons continuously lost energy:
- Their angular velocities would continuously change.
- The frequency of the emitted radiation would also change continuously.
- Therefore, atoms should emit a continuous spectrum.
However, experiments showed that hydrogen emits only certain discrete wavelengths, producing a line spectrum. Rutherford's model could not explain this observation.
Bohr's Modifications (1913)
Bohr combined classical concepts with early quantum ideas and proposed his theory in the form of three postulates.
First Postulate: Stationary Orbits
An electron in an atom can revolve only in certain stable orbits without emitting radiant energy.
- Electrons do not lose energy while moving in stationary orbits.
- Each stationary orbit has a definite energy.
- This postulate explains the stability of atoms.
Second Postulate: Quantization of Angular Momentum
Electrons can revolve around the nucleus only in those orbits for which their angular momentum is quantized.
L = nh / 2π
- L = angular momentum of the electron
- n = 1, 2, 3, ... (principal quantum number)
- h = Planck's constant
- Only certain discrete orbits are allowed.
- Electrons cannot possess arbitrary values of angular momentum.
- The energy of the electron is quantized.
Third Postulate: Emission and Absorption of Radiation
Radiation is emitted or absorbed only when an electron makes a transition from one stationary orbit to another.
hν = Ei − Ef
- h = Planck's constant
- ν = frequency of radiation
- Ei = energy of the initial state
- Ef = energy of the final state
For Emission: Ei > Ef and a photon is emitted.
For Absorption: The electron absorbs energy and moves to a higher orbit.
This postulate successfully explains the line spectrum of hydrogen.
Frequently Asked Questions (FAQ)
1. Why did Rutherford's model fail?
It could not explain atomic stability and the hydrogen line spectrum.
2. What are stationary orbits?
They are stable orbits in which electrons revolve without emitting energy.
3. Why do electrons not lose energy in stationary states?
According to Bohr's first postulate, electrons in stationary orbits do not radiate energy.
4. What is the angular momentum condition?
L = nh / 2π
5. What is the principal quantum number?
It specifies the allowed energy level of an electron.
6. When is radiation emitted?
When an electron moves from a higher energy state to a lower energy state.
7. When is radiation absorbed?
When an electron moves from a lower energy state to a higher energy state.
8. What is the frequency condition?
hν = Ei − Ef
9. How many postulates did Bohr propose?
According to NCERT, Bohr proposed three postulates.
10. How did Bohr explain the hydrogen spectrum?
Electrons occupy specific energy levels, resulting in discrete spectral lines.
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