Which scientist proposed the planetary model, where electrons orbit the nucleus in defined shells?

• A. John Dalton
• B. JJ Thomson
• C. Bohr
• D. Ernest Rutherford

Answer: (C)

Think of atoms as a tiny solar system where electrons orbit the central nucleus in specific, allowed paths. This idea helps explain why atoms emit or absorb light at only certain wavelengths: the electrons can only occupy certain energy levels, so transitions between these levels release or absorb photons with fixed energies.

Niels Bohr proposed this planetary model by introducing quantized orbits for electrons around the nucleus. In his view, electrons move in defined shells with specific energies, and they stay in those shells rather than spiraling inward. When an electron drops from a higher shell to a lower one, a photon is emitted with an energy equal to the difference between the two levels; when it absorbs energy, it jumps to a higher shell. This neatly accounts for the discrete spectral lines seen in hydrogen and similar systems, something classical ideas struggled to explain.

Earlier ideas like Dalton’s simple indivisible atoms, Thomson’s plum pudding model with electrons scattered in a positively charged material, and Rutherford’s nucleus-centered picture without defined electron shells don’t explain those distinct energy levels and spectral lines, so the Bohr model is the one that introduces the defined shells around the nucleus.

While later quantum theory refined the picture, the essential concept — electrons existing in defined shells around a nucleus — comes from Bohr’s model.