From neon to starlight

We know what stars are made of for the same reason a neon sign is red. Electrons around an atom can only hold certain energies, every element has its own set, and when an electron drops from one to another the difference leaves as light of one exact colour. An atom is about a tenth of a nanometre across. It signs its name in light, and the signature still reads after trillions of kilometres.

That has had a hold on me since Chem 1 at university. This week I finally built Orbitals, a lab toy for looking at it.

The symbol is wrong

The drawing at the top of this page is how everyone pictures an atom: electrons circling a nucleus like planets. It has been out of date since 1926. An electron doesn’t follow a path. What it has is an orbital, a region where it’s likely to be found, and orbitals come in shapes. First a sphere, then dumbbells, then clovers, then shapes nobody has a household name for.

Each orbital holds two electrons. They come in sets (one sphere, three dumbbells, five clovers, seven of the next kind), and the sets fill from the lowest energy up. Add a proton and the atom takes on one more electron, which goes into the lowest opening left. Where that last electron lands decides most of how the element behaves. It’s also why the periodic table has the layout it has: each column is a family whose outermost electrons sit in the same arrangement.

Why neon keeps to itself

Neon has ten electrons, and they exactly fill its first two shells. An eleventh would have to start a third shell, far from the nucleus and behind ten electrons that cancel nearly all of its pull. Nothing holds it there. Neon’s own electrons are held hard too: pulling one off costs 21.6 eV. Sodium, one proton later, has that lonely eleventh electron and gives it up for 5.1 eV. That is why sodium fizzes in water and neon reacts with nothing.

One thing to note: “full shell” is shorthand. From argon down, the noble gases have whole sets of empty orbitals in their outer shell. What’s full is the outer sphere and its three dumbbells, eight electrons, and the next opening is a big step up.

How the sign makes its light

A neon sign is a glass tube of thin gas with a few thousand volts across it. Loose electrons race down the tube and slam into neon atoms. An atom doesn’t keep the electron that hit it. Instead, the collision knocks one of neon’s own outer electrons up into a higher, empty orbital, where it can’t stay. Tens of billionths of a second later it drops, and the energy it loses leaves as a single photon.

The size of the drop sets the colour exactly: the wavelength in nanometres is about 1,240 divided by the drop in electron volts. One of neon’s bright red lines is a 1.94 eV drop, which comes out at 640 nm. That drop isn’t the trip home, though. It’s a step between two excited orbitals. The last fall back to where the electron started is about 16.7 eV, far ultraviolet at 74 nm, and you never see it.

A neon sign’s colour comes from the step before the electron gets home.

Hydrogen is the one atom where you can do the sums yourself. With a single electron, the energy of shell n is 13.6 eV divided by n squared. A fall from shell 3 to shell 2 gives 13.6 × (1/4 − 1/9) = 1.89 eV, and 1,240 ÷ 1.89 = 656 nm: the red line astronomers call H-alpha.

Not all light is made this way. A lamp filament, or the surface of the Sun, glows in a smooth rainbow simply because it’s hot and dense. The lines are what loose atoms add to that rainbow, or take out of it.

Reading a star

Taking out is how it works for stars. The body of a star shines in every colour. On the way out, that light passes through cooler gas in the star’s outer layers, and the atoms there absorb exactly the colours they would otherwise emit. What reaches a telescope is a rainbow crossed with thin dark lines, and the lines are a parts list.

Joseph von Fraunhofer mapped hundreds of them in sunlight in 1814 without knowing what they were. In 1859 Gustav Kirchhoff and Robert Bunsen matched them to elements glowing in a lab flame. Then in 1868 astronomers found a yellow line in the Sun’s light, at 587.6 nm, that matched no known element. They named the new element helium, after the Greek word for the Sun.

Helium was discovered 150 million kilometres away, 27 years before anyone found it on Earth.

The rules that govern something too small to see are how we learned what the largest things we can see are made of. I find that symmetry hard to get over.

Try it

Orbitals draws all 118 elements. Step through the table one proton at a time and watch where each new electron goes. On hydrogen and the noble gases, pick a line on the spectrum strip and it plays the fall.

Fair warning: the moving dots are invented, so you can count the electrons and see which orbital each belongs to. Real electrons have no path, which is where this post started. The shapes, the filling order and the wavelengths are real.

More experiments like it are in the lab.