Bohr's Model Postulates for NEET: Key Points, Tricks & MCQs

What NEET Asks

• Direct conceptual questions on individual postulates of Bohr's model.
• Application of angular momentum quantization formula for specific orbits.
• Basis for explaining the stability of atoms and hydrogen spectral lines.

Key Points

• Stationary States: Electrons revolve in fixed circular orbits (called stationary states) without radiating energy.
• Quantized Angular Momentum: Only orbits where the angular momentum (mvr) is an integral multiple of h/2π are allowed (mvr = nh/2π, where n = 1, 2, 3...). Electrons cannot exist between these orbits.
• Energy Transitions: Energy is emitted when an electron jumps from a higher to a lower energy state, and absorbed when it moves from a lower to a higher state. The energy difference ΔE = E₂ - E₁ = hν (where ν is the frequency of radiation).
• Centripetal Force: The electrostatic force of attraction between the nucleus and the electron provides the necessary centripetal force for the electron's motion (kZe²/r² = mv²/r).
• Limitations: Bohr's model is strictly applicable only to single-electron species (e.g., H, He⁺, Li²⁺, Be³⁺).

Must-Know Formula / Reaction

mvr = nh/2π

• m: mass of the electron
• v: velocity of the electron
• r: radius of the electron's orbit
• n: principal quantum number (or orbit number, 1, 2, 3...)
• h: Planck's constant
• This formula represents the quantization of angular momentum, a fundamental postulate.

Common Mistakes

• Students often apply Bohr's model concepts to multi-electron atoms, which is incorrect due to electron-electron repulsions.
• Don't confuse 'stationary states' with electrons being static; they are in motion but not radiating energy.
• Misinterpreting the negative sign in energy calculations; emission leads to a negative ΔE, but the energy of the emitted photon is a positive value.

Rapid Revision

Bohr's model postulates fixed, non-radiating orbits (stationary states) with quantized angular momentum (mvr = nh/2π). Energy changes only occur during transitions (ΔE = hν). It's valid solely for hydrogen and hydrogen-like species.