ca-inspire-chemistry-sampler

C05_024A

Before collision

Electron

Speed

Photon

After collision

Electron’s speed

increases.

Photon’s

wavelength

increases.

λ'

Now imagine trying to determine an electron’s location by “bumping” it with a

high-energy photon. Because such a photon has about the same energy as an electron,

the interaction between the two particles changes both the wavelength of the photon

and the position and velocity of the electron, as shown in

Figure 14

. In other words, the

act of observing the electron produces a significant, unavoidable uncertainty in the

position and motion of the electron. Heisenberg’s analysis of interactions, such as those

between photons and electrons, led him to his historic conclusion. The

Heisenberg

uncertainty principle

states that it is fundamentally impossible to know precisely both

the velocity and position of a particle at the same time.

Although scientists of the time found Heisenberg’s principle difficult to accept, it has

been proven to describe the fundamental limitations of what can be observed. The

interaction of a photon with a macroscopic object such as a helium-filled balloon has so

little effect on the balloon that the uncertainty in its position is too small to measure.

But that is not the case with an electron moving at 6

m/s near an atomic nucleus.

The uncertainty of the electron’s position is at least 10

m, about 10 times greater than

the diameter of the entire atom.

The Heisenberg uncertainty principle also means that it is impossible to assign fixed

paths for electrons like the circular orbits in Bohr’s model. The only quantity that can be

known is the probability for an electron to occupy a certain region around the nucleus.

Figure 14

When a photon interacts with an electron at rest, both the velocity and the position of the

electron are modified. This illustrates the Heisenberg uncertainty principle. It is impossible to know at the

same time the position and the velocity of a particle.

Explain

Why has the photon’s energy changed?

Get It?

Identify

the only quantity of an electron’s orbit that can be determined.

CROSSCUTTING CONCEPTS

Cause and Effect

What empirical evidence did scientists have that

supports the claim that electrons have both particle and wave properties?

Lesson 2 • Quantum Theory and the Atom

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