Physics · Atoms · NEET
An electron moving in a circle is constantly accelerating (centripetal acceleration), even if its speed does not change. Classical electromagnetic theory says any accelerating charge radiates energy as electromagnetic waves. So the electron keeps losing energy, its orbit radius keeps shrinking, and it spirals into the nucleus. The atom would collapse in about 10 to the power minus 8 seconds. But real atoms are stable, so the model is wrong here.
As the electron spirals inward, its orbit radius decreases continuously, so its frequency of revolution increases continuously. Since the radiated frequency equals the revolution frequency, the emitted light frequency would also change continuously. This gives a continuous spectrum (all frequencies). But experiments show atoms emit only a line spectrum (a few sharp, fixed wavelengths). This mismatch is the second big failure.
Yes. In circular motion the direction of velocity keeps changing, so there is always centripetal acceleration pointing toward the nucleus, even at constant speed. Classical theory links radiation to acceleration, not to change in speed. So a steadily orbiting electron still radiates and still loses energy. This is the key idea students often miss.
It correctly said the atom has a tiny, heavy, positively charged nucleus with electrons revolving around it (from the alpha-scattering experiment). What it could not explain was the stability of the atom and the line spectrum. It also said nothing about how electrons are arranged in orbits or their energies. Bohr later fixed these by adding stable non-radiating orbits.
Bohr said electrons revolve only in certain special stable orbits without radiating energy, breaking away from classical theory. Energy is emitted only when an electron jumps from a higher orbit to a lower one, and only a fixed photon energy is released each time. Fixed energy jumps give fixed wavelengths, which explains the line spectrum. This directly repairs both Rutherford failures.
Given below are two statements: Statement I: Atoms are electrically neutral as they contain equal number of positive and negative charges. Statement II: Atoms of each element are stable and emit their characteristic spectrum. In the light of the above statements, choose the most appropriate answer.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
One, it cannot explain the stability of the atom: the orbiting electron should radiate energy, spiral in and collapse. Two, it wrongly predicts a continuous spectrum instead of the observed line spectrum.
The spiralling electron would fall into the nucleus in roughly 10 to the power minus 8 seconds, so the atom could not exist. Real atoms are stable, which proves the model is incomplete.
Classical electromagnetic theory says any accelerating charge emits electromagnetic radiation. The electron in a circular orbit is always accelerating (centripetal), so it continuously radiates and loses energy.
No. It described a tiny nucleus with revolving electrons but gave no rule for the size, energy or arrangement of the electron orbits. Bohr's postulates supplied these missing rules.
No. Its central idea, a small dense positive nucleus with electrons around it, is correct and still used. Only the classical picture of orbiting electrons fails, and Bohr's quantum ideas fixed that part.
