Quantum Theory of the Atom

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Presentation of Lecture Outlines, 7–27

If matter has wave properties, why are they not commonly observed?

The de Broglie relation shows that a baseball (0.145 kg) moving at about 60 mph (27 m/s) has a wavelength of about 1.7 x 10-34 m.

This value is so incredibly small that such waves cannot be detected.

Quantum Mechanics

Slide 28

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Presentation of Lecture Outlines, 7–28

Electrons have wavelengths on the order of a few picometers (1 pm = 10-12 m).

Under the proper circumstances, the wave character of electrons should be observable.

Quantum Mechanics

If matter has wave properties, why are they not commonly observed?

Slide 29

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Presentation of Lecture Outlines, 7–29

Quantum Mechanics

In 1927, it was demonstrated that a beam of electrons, just like X rays, could be diffracted by a crystal.

The German physicist, Ernst Ruska, used this wave property to construct the first “electron microscope” in 1933.

Slide 30

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Presentation of Lecture Outlines, 7–30

Scanning Tunneling Microscope

Slide 31

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Presentation of Lecture Outlines, 7–31

Scanning Tunneling Microscope Electron Clouds

Image Movie

Slide 32

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Presentation of Lecture Outlines, 7–32

Quantum Mechanics

Quantum mechanics is the branch of physics that mathematically describes the wave properties of submicroscopic particles.

We can no longer think of an electron as having a precise orbit in an atom.

To describe such an orbit would require knowing its exact position and velocity.

In 1927, Werner Heisenberg showed (from quantum mechanics) that it is impossible to know both simultaneously.

Slide 33

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Presentation of Lecture Outlines, 7–33

Quantum Mechanics

Heisenberg’s uncertainty principle is a relation that states that the product of the uncertainty in position (Dx) and the uncertainty in momentum (mDvx) of a particle can be no larger than h/4p.

When m is large (for example, a baseball) the uncertainties are small, but for electrons, high uncertainties disallow defining an exact orbit.