Electrons:

The life of the party

 BY SARAH LAUBACHER

Maybe you remember staring at your desk in elementary school thinking, “That’s made of atoms?”  First your teacher tells you everything’s made of atoms. Then you’re told that an atom is about one-millionth the size of a human hair and that atoms are zipping around or vibrating, always in motion. It’s hard to imagine all this atomic activity going on inside your solid, stationary school desk. 

It’s especially difficult to understand how atoms behave when scientists keep changing their minds!  First your teacher shows you the “classical model,” and you think, “OK, that’s pretty easy – electrons orbit the nucleus like planets orbit the sun.”  Well, that’s not exactly correct.  Then your teacher talks about, “the quantum approach to the atom,” and you start freaking out. 

Don’t worry; it’s not too difficult – as long as you can comprehend the characteristics of electrons.  Electrons are perhaps the most charismatic of the subatomic particles.  They’re the socialites you can never track down.  They sort of jump around, they sort of go with the flow, and they’re kind of sneaky about it.  They emit an aura that is attractive to some, repulsive to others, and sometimes not quite either, but mysteriously appealing nonetheless.  Their electrically charged personalities are quite colorful, and they like heavy metal.

Why is it so difficult to pinpoint this party animal’s position?  Consider that an electron is so small that a little photon can knock it around.  Since photons are our tool for locating electrons, that poses a problem. Scientists bounce photons off electrons to measure their position: by figuring out the photon’s change in momentum, they figure out where the electron is.  But on impact the photon transfers momentum to the electron, which moves it to a different location. When the photon bounces back, the electron isn’t there any longer, so the scientist is ignorant of the electron’s new location. This is just one example of how their small size can complicate experiments. 

To further appreciate the difficulty, consider Heisenberg’s uncertainty principle. The principle says that it is impossible to measure the position and momentum of a particle with infinite precision.  The idea applies to the measurement of small objects, and with about 1/2000 the mass of a proton, electrons are definitely small objects.

Not only does their small size make it difficult to discern where they are, but also what they are – particles or waves?  You could say they are particles with “wave-like” qualities. This split personality helps them get through difficult situations – high-energy barriers, for instance.  Electrons love metal, and feel most comfortable traveling along metal conductors.  But because of this particle-wave duality, they can leak through the “barrier” that faces them when their conductor is cut – as long as the gap is only about a nanometer.  (This quantum-mechanical concept is called “tunneling,” and is used in nanotechnology research.)

Another example of quantum behavior comes up when we start to look at electrons as occupying shells around the nucleus, instead of orbits in concentric circles around a center. Scientists see them like electron clouds, and refer to them as occupying “orbitals” or “states.” Remember that the nucleus is composed of positive protons and neutral neutrons, resulting in a net positive charge. You probably know that electrons have a negative charge, so you may ask yourself, “If opposites attract, why don’t electrons get sucked into the center to be with the protons? And if they’re all close to each other in these clouds, what keeps them from feeling ‘repulsed’ and flinging out of the atom altogether?” 

Well, the forces keeping everything intact are a little complicated. Along with the “opposites attract/likes repel” idea, another principle comes into play: no two particles, (such as electrons) can occupy the same state at the same time. It is called the Pauli exclusion principle – exclusion because one particle can exclude all the others from being in its state. Even when electrons seem to be in the same state, or orbital, they really aren’t because they have opposite spins.

Their states can also change when they get energized by light or collisions.  When they get excited like this, they jump to higher energy orbitals. On the way back down, they emit energy in the form of photons, which produces various colors – a pretty neat party trick.

Hopefully you are now convinced that electrons are the most exciting socialites at the subatomic shindig.  With these concepts under your belt, you can begin to study electrons on the dance floor, as they use their angular momentum to spin up, spin down, solo and in pairs.