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lesson 02 / light from atoms

Every element has its own barcode.

Heat some sodium and it glows yellow. Heat copper and it glows green. Not random: the colours come from electrons dropping between energy levels. Scroll down and I'll show you how.

↑ the real hydrogen lines (656, 486, 434, 410 nm)

ground state

1

Electrons live on levels

In the Bohr model, an electron can only sit on certain energy levels (shells), called n = 1, 2, 3… Think of a ladder: you can stand on a rung, never between two. The lowest one, n = 1, is the ground state.

2

Give it energy, it jumps

Heat it in a flame or pass electricity through the gas, and the electron absorbs energy and jumps to a higher level. Now it's excited. It doesn't stay there long.

3

It falls back and lets out light

When it drops back down it gives the extra energy away as a tiny packet of light, a photon. The energy of the photon = the gap between the two levels. Nothing more, nothing less.

4

Bigger drop, bluer light

A big drop gives a high-energy photon (violet/blue). A small drop gives a low-energy one (red). So the colour tells you how big the jump was.

5

Only certain gaps = only certain colours

The levels are fixed, so only a few drops are possible. That's why we see separate bright lines on a black background instead of a rainbow. That pattern is the line emission spectrum.

6

Lines get closer at the top

Higher up, the levels squeeze together, so the lines bunch up too. Eventually they merge (the convergence limit). Past that, the electron has left the atom completely.

lab A

Make a hydrogen line

Pick where the electron starts, then press drop. In hydrogen, falls that end on n = 2 give the four colours we can see. Falls to n = 1 are ultraviolet (invisible) and falls to n = 3 are infrared.

land on:

pick a starting level on the ladder

compare

Rainbow vs. barcode

Drag the slider. On the left: white light from a hot solid (like a bulb filament), which gives every colour, a continuous spectrum. On the right: a hot gas, which only gives its own lines.

continuous
hot solid or liquid, all colours
emission (line)
hot gas, bright lines on black
absorption
white light through cool gas: dark lines at the same spots
lab B

Flame test bench

Dip a clean wire loop in a metal salt and hold it in a blue Bunsen flame. Each metal colours the flame because of its own electron drops. Tap a sample.

no sample yet, flame is just blue

lab C

Who's in the mystery gas?

An astronomer points a spectroscope at a glowing gas cloud. Compare its barcode with the reference ones. Every line in the reference must appear in the mystery, so a mix shows both sets.

mystery sample

real life

Where you've already seen this

Fireworks

Strontium salts → red, sodium → yellow, copper → blue-green. Same chemistry as the flame test.

Orange streetlights

Low-pressure sodium lamps glow that famous orange-yellow at 589 nm.

What stars are made of

Helium was found in the Sun's spectrum in 1868, before anyone found it on Earth.

Neon signs

"Neon" signs are tubes of gas with electricity through them. Real neon glows red-orange.

The 20-second version

  1. Electrons sit on fixed energy levels.
  2. Energy in → electron jumps up (excited).
  3. Falls back → gives out a photon. Energy of photon = size of gap.
  4. Fixed gaps → only certain colours → line spectrum.
  5. Each element's lines are unique, like a fingerprint.

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