The Bohr model of the atom proposed the existence of?

Step 1: Analyze the question We are asked what the Bohr model specifically introduced or proposed about the atom.

Step 2: Explain the concept Different atomic models added different ideas:

• Dalton: atoms are indivisible solid spheres.
• Thomson: the "plum pudding" model, with electrons embedded in a positive sphere.
• Rutherford: a small, dense, positively charged nucleus at the center, with electrons around it.
• Bohr (1913): electrons move in fixed circular paths called orbits, or energy levels (shells), around the nucleus. Electrons do not radiate energy while in these orbits. They absorb or emit energy only when they jump between levels.
• Chadwick (1932): discovered the neutron.

Step 3: Check the options

• A. The nucleus: This was Rutherford's discovery. Bohr built on it, but did not propose it.
• B. Electron shells: This is Bohr's key contribution, since electrons occupy specific quantized energy levels.
• C. Nucleons: This is a general term for protons and neutrons, and it is not part of Bohr's proposal.
• D. Neutrons: These were discovered by Chadwick after Bohr's model.

Step 4: Answer The correct option is B. electron shells.

Tip: Link each scientist to one keyword:

• Thomson → electron
• Rutherford → nucleus
• Bohr → shells (energy levels)
• Chadwick → neutron