Tunneling
Definition and meaning of Tunneling in chemistry.
Tunneling is a quantum mechanical phenomenon in which a particle passes through an energy barrier despite having insufficient energy to classically overcome it. The particle exhibits a nonzero probability of appearing on the opposite side of the barrier due to its wave-like properties.
In more detail
Tunneling arises from wave-particle duality and the Heisenberg uncertainty principle. The tunneling probability decreases exponentially with barrier width and height, and increases with the particle's energy and inversely with its mass. Tunneling is essential in radioactive decay (alpha particles escaping nuclei), semiconductor electronics, enzyme-catalyzed reactions, and nuclear fusion in stars. Without tunneling, many chemical and nuclear processes would proceed at unmeasurably slow rates at room temperature.
Key facts
| Field | Physical Chemistry |
|---|---|
| Based on | Wave-particle duality and Heisenberg uncertainty principle |
| Probability depends on | Barrier width, barrier height, particle mass, and particle energy |
| Key applications | Radioactive decay, semiconductors, enzyme catalysis, STM microscopy |
In alpha decay of uranium-238, alpha particles tunnel through the repulsive Coulomb (electrostatic) barrier surrounding the nucleus and escape, allowing spontaneous radioactive emission even though the alpha particles lack sufficient classical energy to overcome that barrier. The strong nuclear force creates the attractive potential well that initially confines the alpha particle, while tunneling lets it escape through the surrounding electrostatic barrier.
Frequently asked questions
Why don't macroscopic objects tunnel through walls?
Macroscopic objects have extremely large mass, making their de Broglie wavelength vanishingly small and the tunneling probability essentially zero.
Does tunneling violate energy conservation?
No. The particle temporarily borrows energy from quantum fluctuations permitted by the uncertainty principle; energy is conserved over the timescale of the tunneling event.