Which principle states it's impossible to know exact position and momentum of a particle simultaneously?

• A. Aufbau Principle
• B. Heisenberg Uncertainty Principle
• C. Atomic Radius
• D. Ionization Energy

Answer: (B)

This question tests the Heisenberg Uncertainty Principle: you cannot know exact position and momentum of a particle at the same time. In quantum mechanics, position and momentum are tied to the particle’s wavefunction. The more precisely you pin down where the particle is (making Δx small), the less precisely you can know how fast it’s moving (making Δp large), and vice versa. This fundamental limit is expressed as Δx Δp ≥ ħ/2, meaning nature itself imposes a trade-off, not just a measurement flaw. The intuition is that a precise position measurement uses a probe with a broad range of momenta (short wavelength), which perturbs the particle’s momentum, while a momentum measurement uses a long-wavelength probe, leaving the position ill-defined. Other concepts refer to different ideas: orbital filling, atomic size, and the energy required to remove an electron, none of which capture this intrinsic limit on simultaneous measurements.