A: Physicists faced the same question at the end of the 19^th century. Classical physics says electrons should crash immediately. Therefore, atoms shouldn’t exist. But, they do.

[Kenneth Snelson] Bohr-de Broglie atom model: one electron standing wave fits into the first shell, like a snake grabbing its tail, the second electron wave exhibits two standing waves, the third, three. . .

A classicist gets stumped this way: Maxwell’s equations of electromagnetism say that accelerating electrical charges emit electromagnetic waves. The electrons are accelerating because they whiz around in circular paths, constantly changing direction. So, the electrons must emit electromagnetic waves (light) and thereby lose energy. Then, like a satellite losing energy through atmospheric friction, they must spiral into the nucleus—in a split second!

Maxwell’s equations do hold. So, what’s wrong? The Universe is grainy, not continuous. It comes in discrete chunks or "quanta." We extended physics and solved the dilemma.

In 1905, Einstein declared that a light wave’s energy is not spread continuously over the wave. Instead, it is concentrated in chunks called photons. Moreover, a photon’s energy can take on only discrete values that depend on the photon’s vibration frequency.

In 1913, Neils Bohr used Einstein’s quantum idea to devise a better atom model. Bohr said that only discrete electron orbits are allowed—the ones corresponding to the discrete values of the electron’s energy.

He stated (without proving) that electrons in allowed orbits don’t radiate light. That insight solved the problem. Electrons won’t lose energy by radiating light and, therefore, won’t crash into the nucleus.

About ten years later, Louis de Broglie added the missing element to Bohr’s picture. Electrons are not merely discrete particles but are also waves—just like light. An astounding notion! First, Einstein tells us that light is both a wave and a particle. Now de Broglie says that mass is both a particle and a wave. The electron’s charge and mass are not concentrated in a particle, but rather smeared into a wave wrapped around the nucleus. Most important: the wave has the right wavelength to form standing waves. See figure of atom model above.

The trick is standing waves. Think of a violin string vibrating between the constraints of two frets. The vibrating string forms standing waves of sound—notes. The string only vibrates in discrete states: the fundamental, first overtone, second overtone, etc.

The electron wave also oscillates in a confined space—its orbit or shell—and forms standing waves. The wave must follow a circular path (the orbit). Furthermore, the wave must fit inside the orbit—evenly! Otherwise, the wave would destroy itself by wavelike interference. This sets up a special energy condition (called an energy quantum state or eigenstate) for the electron wave so that the wave persists and does not radiate energy, says Rod Nave, physicist at Georgia State University. No light radiates, no energy is lost, and the electron doesn’t crash into the nucleus. Pretty clever, eh?

Kenneth Snelson: Atom

Jefferson Lab: How many protons, electrons, neutrons in an atom?

Jefferson Lab: How to make an atom model

Jefferson Lab: Periodic Table

Q: What color are humans most sensitive to---green, red, and blue? Irene C.

[Corel] Colors humans are most sensitive to—green, red, blue.

A: Right on. Human eyes absorb green, red, and blue light best. That’s why they are called the "primary colors." Birds see red best; bees can’t see red but do see ultraviolet light— beyond our sensitivity range. A bee sees a blue flower when we see a yellow one. Dogs see mostly gray, barely perceiving colors.

WonderQuest: Primary colors

WonderQuest: Color— all in the eye of the beholder

Q: Can you help me find a toy zunzuncito bird or a poster that would help elementary children learn about this bird? —Debbie H.

I found a good picture for use as a poster. See "Further Surfing" below.

The zunzuncito is the world’s smallest bird—a blue hummer that flits among the flowers of Cuba. It’s just bigger than a grasshopper: a scant 2.5 inches (6 cm) and weighs a little more than four paper clips ( 0.07ounces, 2 g). The name—zunzuncito—means (loosely) "little buzz buzz" in Spanish. Like all hummingbirds, it hovers while eating, drinking, and collecting nest material. It copulates and nearly exists in flight.

By the way, hummingbirds have the largest brain relative to their size of all birds.

A picture: The Orchid Lady: Cuban Hummingbird

Elbert Creer: The bumblebee hummingbird

(Answered April 25, 2003)