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💡 The rescue
Niels Bohr saved the atom from collapse with one bold idea: electrons are allowed to live only in certain fixed shells, where they simply do not lose energy.
How does Bohr’s model explain stability?
- Electrons move only in fixed circular shells (stationary states / orbits).
- In a stationary state , an electron’s energy stays constant — it does not lose energy.
- So the electron does not spiral in, and the atom stays stable .
What are energy levels (shells)?
Shells (Energy Levels)
Fixed circular paths K, L, M, N where electrons revolve; energy increases outward from the nucleus. K (n equals 1) is closest and lowest in energy
- Fixed circular paths named K, L, M, N… (or n = 1, 2, 3, 4…).
- Each shell has a definite energy , so shells are also called energy levels .
- Energy increases as we move outward ; K (n=1) is closest and lowest.
- An electron jumps shells by absorbing or releasing a fixed amount of energy.
👤 Meet a Scientist — Niels Bohr
- Professor of physics at Copenhagen University, Denmark.
- Explained how electrons stay around the nucleus without collapsing.
- Won the Nobel Prize (1922) for his work on the structure of the atom.
🧵 Threads of Curiosity
- Why K, L, M, N… and not A, B, C, D?
- The names came from early X-ray experiments.
- Charles Barkla called the first X-ray line “K”.
- He left room for an earlier series before K.
- No such earlier series was ever found.
- Bohr adopted the same notation for atomic shells.
NCERT Question 2 — Which of the following statements
Which of the following statements are correct or incorrect according to the Bohr’s atomic model? Give a reason for each statement. (i) Electrons lose energy while moving in fixed orbits and slowly fall into the nucleus. (ii) Electrons can exist anywhere around the nucleus with no fixed energy. (iii) Electrons revolve around the nucleus in orbits of fixed energy without losing energy. (iv) Electrons can be found between energy levels as they move around the nucleus.
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NCERT Question 6 — Electrons move around the nucleus
Electrons move around the nucleus in orbits. Why do they not fly away from the atom? Explain what keeps them attracted to the nucleus.
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NCERT Question 10 — Both Rutherford's and Bohr's models
Both Rutherford’s and Bohr’s models have electrons orbiting the nucleus. Why did Rutherford’s model fail to explain atomic stability, while Bohr’s model succeeded?
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✅ Test Yourself
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Who discovered the electron, and in which year?
Show Answer
J. J. Thomson, in 1897, from his cathode ray experiments. -
Why is Thomson’s model called the plum pudding model?
Show Answer
Electrons sit embedded in a sphere of positive charge, like plums in a pudding. -
What did the few sharply deflected α-particles prove?
Show Answer
That the positive charge and almost all the mass sit in a tiny, dense nucleus. -
Which model finally explained why atoms are stable?
Show Answer
Bohr’s model — electrons in fixed shells (stationary states) do not lose energy.
Important Definitions
- Stationary states — fixed circular shells (also called orbits or energy levels) in which an electron does not lose energy.
- Energy levels — shells named K, L, M, N whose energy increases as they get farther from the nucleus.
Comparing the Atomic Models
| Model | Scientist | Key idea | Main limitation |
|---|---|---|---|
| Dalton | John Dalton | Atom is a solid, indivisible particle | Could not explain subatomic particles |
| Thomson | J. J. Thomson | Electrons embedded in a positive sphere (plum pudding) | Failed the gold foil experiment |
| Rutherford | Ernest Rutherford | Tiny dense nucleus; electrons revolve around it | Could not explain atomic stability |
| Bohr | Niels Bohr | Electrons in fixed energy levels (shells) | Refined later by the quantum model |
