Who proposed the planetary model of the atom with electrons orbiting the nucleus?

Step 1: Analyze the question The question asks who proposed the planetary model, in which electrons orbit a central nucleus like planets around the sun.

Step 2: The concept (history of atomic models)

• J.J. Thomson (1897-1904): "Plum pudding" model. Electrons are embedded in a positively charged sphere. There is no nucleus.
• Ernest Rutherford (1911): Gold foil experiment. Most alpha particles passed straight through, and a few were deflected strongly. This showed that the atom has a tiny, dense, positively charged nucleus with electrons orbiting around it. This is the nuclear (planetary) model.
• Niels Bohr (1913): Improved Rutherford's model by adding quantized energy levels (shells). Electrons orbit only in fixed energy levels.
• Erwin Schrödinger (1926): Wave-mechanical (quantum or electron cloud) model with orbitals.

Step 3: Solution The planetary model with electrons orbiting a nucleus was first proposed by Rutherford after his gold foil experiment. Bohr refined it by adding fixed energy levels, but the original planetary picture is credited to Rutherford. Thomson had no nucleus, and Schrödinger's model uses orbitals (probability regions), not orbits.

Step 4: Answer The correct option is B. Ernest Rutherford.

Exam tip: Remember the order T-R-B-S (Thomson → Rutherford → Bohr → Schrödinger), along with the key word for each one:

• Thomson = plum pudding
• Rutherford = nucleus (gold foil)
• Bohr = energy levels
• Schrödinger = orbitals/wave

If the question says "nucleus discovered" or "gold foil", think Rutherford. If it says "energy levels or shells", think Bohr.