Have you ever looked up at a fireworks display and wondered where all those colors are actually coming from? To answer this question we must first go back to atomic theory.
As you may recall, an atom is comprised of electrons, protons, and neutrons. The protons and neutrons are contained within the nucleus while the electrons exist in discrete energy levels outside of the nucleus. When an atom is exposed to enough energy, such as the heat created within a firework explosion, an electron can absorb this energy and get promoted to a higher energy level. Shortly after, the electron will fall back down to a lower energy level, releasing energy in the form of light. Therein lies the key to colorful fireworks!
The wavelength, or color, of light depends on the energy that is released (i.e. the energy difference between the electron’s two energy levels). Light that can be seen with the human eye, also known as visible light, has wavelengths between 750 and 380 nanometers. (Nanometers are small – there are 1,000,000,000 nanometers in 1 meter!) As the wavelength shortens within this range, the colors transition through the commonly known rainbow sequence starting with red and ending with violet; as illustrated in the visible light spectrum below.
Now if we think back to our picture of the atom, the electrons are in discrete energy levels, meaning that a transition between the two different energy levels will produce light of only one wavelength rather than a continuous spectrum, like the one above. For example, if the energy difference between the two levels is 2.03 eV it would emit orange light (610 nm). As it turns out, the wavelength and energy are inversely proportional, so a transition with a greater energy of 2.33 eV would produce light of a shorter wavelength, 532 nm or green light.
Every element has a unique number of electrons and electron configuration which means not every element will contain electronic transitions of the same energies – giving each element its own “spectral footprint.” This fact is commonly used in laboratories to identify elemental compositions of unknowns. You may have even done this yourself before in science class to identify metals using flame tests! Today, scientists are using these “spectral footprints” to determine the compositions of rocks on Mars as explained in a previous post by ScIU blogger Ed Basom.
Beyond using these electron excitations and subsequent relaxations to identify unknowns, we can also use them to strategically design materials that emit colorful light, such as those used in fireworks or neon signs. As it turns out, the brilliant colors we see in fireworks are due to just a handful of salts (a metal-nonmetal compound, such as calcium chloride). See below for the list of compounds responsible for each color:
Red: strontium salt or lithium salt
Orange: calcium salt
Yellow: sodium salt
Green: barium salt
Blue: copper salt
Purple: copper salt and strontium salt, potassium salt, or rubidium salt
On the 4th, as you kick back and relax with family and friends feel free to share the chemistry of fireworks with everyone. Have a Happy Fourth of July! I hope you all enjoy those brilliant electron relaxations!