A Bowl of Electrons
Like everything else in the universe, the stuff we eat is made of atoms. An atom is like a little solar system, with a nucleus at its center. Electrons orbit the nucleus like planets circling a sun. (Although actually, according to quantum mechanics, you can’t know exactly where an electron is at any moment—and so really this orbit is less of a fixed path than a sort of cloud of possible positions.) There might be one electron or several within any given atom; they orbit at certain typical distances, known to chemists as orbital shells. Only a finite number of electrons can occupy an orbital shell at any one time: two in the first shell, eight in the second, eighteen in the third, thirty-two in the fourth, and so on—a pattern that defines how the rows of the periodic table are laid out. All of chemistry depends on the fact that electrons that aren’t part of fully filled shells are less stable, especially as they get farther from the nucleus. It’s as if an electron is not meant to wander too far from home.
From time to time, something bumps into an atom. If it’s a photon—a particle of light—then energy from the collision knocks an atom’s electrons into orbits that are more distant from the nucleus. These “high-energy” electrons are like marbles poised on the lip of a bowl—they want to release their potential energy by rolling back down toward the center or, if another atom is near, by spilling over into itsbowl. Which way they fall depends on the precise balance of instabilities in each atom—in other words, which has the shell most desperate to be filled. When an atom poised to give up an energetic electron gets close to a neighbor eager to take it, that electron rolls from the lip of one bowl down into the other. In falling, it releases energy. However abstract this may seem, it is the very essence of life. Photons careening from the sun bang into electrons in chlorophyll in plants; a series of chemical reactions transfers those energized electrons from one atom to the next, until eventually they are stored up inside the sugars or starches in fruits, stalks, and seeds.
Quite possibly the most beautiful description of electron transfer that I’ve ever read.