🌊 Module 2 · Lesson 2/3

🪜The Atom's Energy Ladder

Why neon signs glow in pure colours — and how we know what stars are made of.

⏱️ 15 minStart

🎯 By the end you will…

  • ✓Explain energy levels in an atom
  • ✓Connect jumps between levels to light colours
  • ✓Understand spectral “fingerprints”

Pass the light of a glowing gas through a prism and you don't get a rainbow — you get a few sharp, bright lines. Hydrogen always shows the same lines, neon shows its own, sodium its own. Each element has a unique spectral fingerprint.

In 1913 Niels Bohr explained why: electrons in an atom can only sit on certain energy levels, like the rungs of a ladder. When an electron drops from a higher rung to a lower one, the atom releases exactly one photon carrying the difference in energy.

🎛️Interactive experiment

Click “Excite” to kick the electron up, then watch it fall. Each fall emits a photon — its colour depends on which rungs it jumps between. The spectrum strip collects the lines.

🧮Equation, decoded

Energy levels of hydrogen (Bohr model)

What each symbol means

  • Energy of rung number n
  • The rung number: 1, 2, 3… (only whole numbers!)
  • The energy needed to rip the electron out of hydrogen. eV (electron-volt) is a tiny energy unit
  • Negative means “trapped”: you must add energy to set it free

📖 Say it like a story

Rungs get closer together as you climb (1/n² shrinks fast). Falling from rung 3 to rung 2 releases 13.6 × (1/4 − 1/9) ≈ 1.9 eV — that's the famous red line of hydrogen at 656 nanometres.

🌍Where you meet it

🌌

Astronomy

Spectral lines reveal what distant stars and galaxies are made of.

🎆

Fireworks

Strontium gives red, copper blue-green, sodium yellow.

🏮

Neon signs

Each gas glows with its own set of energy jumps.

✅ Check your understanding

Answer all questions to complete the lesson and earn XP.

  1. 1. Why does each element emit only certain colours?

  2. 2. What happens when an electron falls to a lower level?

  3. 3. Helium was first discovered…

📚 Sources & further reading