Why Are Fireworks Different Colours? It's the Electrons

Why are fireworks different colours? Because every colour in a firework comes from a metal. Strontium gives red, sodium gives yellow, barium gives green, copper gives blue-green. The heat of the explosion gives the electrons in those metal atoms extra energy. As they lose it again, they give it out as light, and the exact colour depends on how far each electron falls. Each element has its own set of falls, so each one has its own colour.

That's the short answer. The longer one is one of the most important ideas in chemistry, so it's worth slowing down.

Electrons live on a ladder, not a ramp

You've probably drawn an atom with electrons in neat circles around the nucleus. Those circles are electron energy levels (or shells), and the key fact about them is that an electron can only sit on a level, never in between. Think of a ladder: you can stand on the third rung or the fourth, but not on the gap.

Normally every electron is as low down the ladder as it can go. That's the atom's ground state. Give an electron exactly the right amount of energy and it jumps up to a higher rung. It's now excited.

It doesn't stay there long. Within a tiny fraction of a second it drops back down, and the energy it had gained comes out as a single packet of light: a photon.

Why a fall gives one exact colour

The energy of the photon is exactly the energy gap between the two rungs. The colour of light is set by its energy:

• a big drop gives a high-energy photon: blue or violet, or even ultraviolet you can't see;
• a smaller drop gives a lower-energy photon: orange or red, or infrared.

In sodium, the most common fall releases about 2.1 electron-volts. That's light with a wavelength of 589 nanometres: the orange-yellow you see in old street lamps, and the yellow flash when salty water boils over onto a gas hob.

Because the rungs are in fixed places, the falls are fixed too. Sodium will always give 589 nm; it can't give 600 nm, because there's no pair of rungs 2.07 eV apart in a sodium atom.

Why every metal is different

The spacing of the ladder depends on the atom: how many protons pull on the electrons, and how many other electrons get in the way. So every element ends up with its own pattern of gaps, and its own set of colours.

Metal	Flame colour	Where you'll see it
Lithium	Crimson red	Red fireworks
Sodium	Yellow-orange	Street lamps, salt in a flame
Potassium	Lilac	A pale purple flame, easily hidden by sodium
Calcium	Orange-red	Brick-red flame tests
Strontium	Bright red	Most red fireworks and flares
Barium	Apple green	Green fireworks
Copper	Blue-green	Blue and green fireworks

Firework makers pack the right metal salts into each shell. Blue is famously the hardest colour to get, because copper compounds tend to break down at the temperatures that make other colours bright.

Flame tests: chemistry's oldest ID card

Chemists used this long before fireworks were understood. Dip a clean wire loop in a salt, hold it in a Bunsen flame, and look at the colour. Yellow means sodium; lilac means potassium; brick red means calcium. The flame test colours are the same ones in the table above.

Push the light through a spectroscope (a prism or a diffraction grating) and you see that the "one colour" is really a few sharp, bright lines on a dark background. That's an emission spectrum, and it works like a barcode. Helium was discovered this way: astronomers saw a yellow line in sunlight in 1868 that didn't match any element known on Earth.

What if you give the electron too much energy?

If an electron gets more energy than it needs to reach the top of the ladder, it doesn't stop on a rung. It leaves the atom completely. That's ionisation, and the minimum energy needed is the ionisation energy. For a hydrogen atom in its ground state it's 13.6 eV. What's left behind is a positive ion.

That links fireworks to something you'll meet again and again in chemistry: why sodium forms Na⁺ so easily, which is the start of ionic bonding. Its outer electron sits alone on a high rung, far from the nucleus, and only needs about 5 eV to escape. The next one needs about nine times as much.

Frequently asked questions

How do electrons give off light?

An electron gives off light when it drops from a higher energy level to a lower one. Heat or electricity first lifts it up to the higher level. When it falls back, the energy it loses leaves as one photon, and the photon's energy, and so its colour, equals the gap between the two levels. Bigger gaps give bluer light; smaller gaps give redder light.

Which metal makes blue fireworks?

Copper, usually as copper chloride. Blue is the hardest firework colour to make, because copper compounds break down at the very high temperatures that make other colours bright. If the shell burns too hot, the blue washes out to white, so firework makers use carefully balanced mixtures to keep the flame just cool enough.

What is a flame test used for?

A flame test identifies which metal ion is in a compound. You dip a clean wire loop into the sample, hold it in a hot Bunsen flame and note the colour: yellow-orange for sodium, lilac for potassium, brick red for calcium, green for barium, blue-green for copper. It works because each metal's electrons have their own energy gaps. You can count an atom's shells for yourself in the Atom Builder.

See it for yourself

In the Electron Lab you can fire photons at a hydrogen atom and watch the electron jump, but only when the energy matches a gap exactly. Switch on "heat the gas" and its falls build up hydrogen's real spectrum line by line: red at 656 nm, then cyan and violet. The flame-test mode lets you identify mystery metals by their colour and lines, and the ionisation mode shows the big jumps that reveal an atom's shells.