Emission spectra: the line pattern of each element
A heated gas does not glow in every colour. It gives out a few sharp lines, and their wavelengths identify the element.
Gas tube
Drag the slider (or the picture) to put a diffraction grating in front of the tube.
Scroll ↓
What you'll be able to do
describe the atom: nucleus in the middle, electrons in shells (energy levels);
explain why heated atoms give out light, in four steps;
tell continuous, emission and absorption spectra apart;
describe a flame test and name the colours of common metal ions;
identify elements from their line spectra.
Takes about 20 minutes. Stuck on a word? Tap Glossary in the bottom corner.
01 · Quick recap
A tiny nucleus, with electrons on shells
From our atomic theory notes: a tiny, dense, positive nucleus (protons + neutrons), with electrons in shells around it. Bohr (1913) said those shells are fixed energy levels.
Shells fill from the inside: 2, then 8, then 8.
Sodium (11 e⁻) = 2,8,1.
An electron can sit on a shell, never between.
Analogy: a ladder. You can stand on a rung, not in the gap.
3D needs WebGL. The animations below show the same idea.
Drag to spin. Press the button to excite the outer electron.
02 · Why atoms glow
Step 1. Ground state
Start at the bottom. The electron sits on the lowest level it can reach, and the atom is stable.
Step 2. Energy in
Now add energy: a Bunsen flame, an electric spark or a beam of light. The electron absorbs just the right amount and jumps up.
Step 3. Excited state
Up here the atom is excited. It's unstable, so the electron only stays for a tiny fraction of a second.
Step 4. Falls back and gives out light
On the way down it gives the energy back as light. Each drop sends out one photon, and the gaps are fixed, so you only get certain colours. That's why we see lines.
Keep scrolling and watch every drop flash.
The whole idea in one line: take in energy → jump up → fall back → light comes out. A bigger drop gives out more energy, so the light is bluer.
03 · Your turn
Drag the electron and make some light
This is a hydrogen atom. Pull its electron up a few rungs, then drop it lower. Every fall shoots out a photon in hydrogen's real colours.
Electron on n = 1. Drag it up.
Tip: drops that end on n = 2 are the visible ones.
04 · Through a prism
Bulb light or gas light?
A prism bends each colour by a different amount, so it fans light out. Compare an ordinary bulb with a glowing gas.
White light → every colour, no gaps: a continuous spectrum.
05 · In the lab
Flame test lab
The practical we do in class: put a metal salt in a hot flame and the flame changes to a colour that tells you the metal.
Clean a nichrome wire loop: dip in concentrated HCl, hold in a blue Bunsen flame until no colour shows.
Dip the loop in the solid metal salt.
Hold it at the edge of the blue (roaring) flame.
Record the colour.
Common mistakes Dirty wire: sodium's yellow hides everything. Mixtures: one colour masks the others. Potassium's lilac is faint, so view through blue glass.
Pick a salt to dip the loop.
Ion
Flame colour
Li⁺
crimson red
Na⁺
yellow-orange
K⁺
lilac
Ca²⁺
orange-red
Cu²⁺
blue-green
Ba²⁺
apple green
Sr²⁺
red
06 · Spot the difference
Three kinds of spectrum
Continuous
All colours, no gaps. Hot solid: bulb filament, the Sun's surface.
Emission
Bright lines on black. Hot gas giving out light.
Absorption
Dark lines on a rainbow. Cool gas in front of white light takes out its own colours.
Continuous
Emission
Absorption
Same element → dark lines sit exactly where the bright ones were.
07 · Fingerprints
Each element has a unique set of lines
No two elements share the same pattern of lines, so a spectrum identifies the element. Watch them glow.
08 · Detective work
Mystery sample
Compare these lines with the barcodes above and name the gas. Level 3 takes the scale away, so you'll have to go by the pattern.
08b · Looking up
Mystery star
We can't visit a star, but its light gives it away. The cooler gas around it takes out dark lines. Which elements are there?
Optional: the maths behind the colours
Photon energy = the gap: E = Ehigh − Elow = hf, and c = fλ. So bigger gap → higher frequency → shorter wavelength.
Example: H red line, λ = 656 nm → f = 3.00×10⁸ ÷ 6.56×10⁻⁷ = 4.57×10¹⁴ Hz. Convert nm to m first.
09 · Test yourself
15 quick questions
If you only remember five things
Nucleus in the centre; electrons in shells = fixed energy levels (Bohr).
Absorb energy → excited → fall back → photon of one exact colour.
Emission: bright lines on black. Absorption: dark lines on a rainbow, same places.