Where Is the Electron? — The Cloud That Fills the Pitch

A cloud, not a ball

Board 1 of the element tool blows an atom up to a football pitch with a grain-of-rice nucleus. The picture is good for showing how empty an atom seems, but it invites one mistake: imagining the electron as a small ball rolling somewhere on the pitch.

An electron is not a ball at one spot but a cloud across the whole pitch. Where the cloud is dense you are likely to find it; where thin, unlikely. The 'rooms' of board 6 are the shapes of this cloud, orbitals: ball, dumbbell and cloverleaf are ways the cloud can lie.

Before measuring, no single place

Perhaps the electron is really at one point and we simply do not know which? An experiment says otherwise: the double-slit experiment.

Cut two narrow slits in a wall and fire electrons one at a time. Balls would pile up in two stripes behind the slits. Instead, once enough single electrons have landed, the screen shows many stripes: the pattern two overlapping ripples make. In 1989 Akira Tonomura's team at Hitachi filmed the pattern building up dot by dot from electrons sent one at a time.

Each electron in effect went through both slits and overlapped with itself. Check which slit it used and the pattern vanishes, leaving two stripes. So physicists say: before it is measured, an electron cannot be said to be at one place. How to interpret this is still argued; the result itself is the same for everyone who runs it.

What is waving?

Sound is air shaking; an ocean wave is water shaking. What shakes when the electron behaves as a wave? This is the hardest part.

The most precisely confirmed theory in physics puts it this way: an electron field fills the whole universe, and one electron is one lump of ripple in that field. The field is the whole sea; an electron is one wave on it. Nothing separate, like air, is doing the shaking; the field itself shakes. That is why the electron is a wave yet always counts as one, two, three.

A little further in

Chemistry happens in the empty space

An almost empty pitch is not useless. The cloud in that space is the atom's outside, and two clouds overlapping is a chemical bond. In water, hydrogen and oxygen do not hand electrons over; they share the region where their clouds overlap: a covalent bond. That is why on board 4 the shared room sits between the two atoms.

When you press a desk, your hand does not pass through, and not because nuclei collide. The electron clouds of hand and desk push each other away. Everything we touch is, in fact, the surface of a cloud.

Pin the place, lose the speed

Measure where an electron is very precisely and how fast it is going blurs; measure its speed precisely and its place blurs. This is the uncertainty principle. It is not a failing of instruments; nature does not give both sharply at once.

It is also why atoms do not collapse. If the electron fell onto the nucleus its place would be too exact, its speed would become enormous and it would fly back out. The cloud sits where the pull inward and this push outward balance. That tug of war sets the size of an atom.

Tool From the Atom to the Periodic Table Blow an atom up to a football pitch and zoom to the grain of rice at its centre, change protons, neutrons and electrons one at a time, count each element's hands to see why water is H2O, then fold the line into the table and sing through all 118. (Korean only)

The question that remainsIf everything we touch is the surface of a cloud, what have we done when we say we touched something?