Electrons put on a show

By Associate Professor Allan Blackman

This article was orignally published in the Otago Daily Times on Tuesday 6 November 2007.

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Every year at about this time, for some reason that I cannot fathom, we celebrate the torture (sanctioned by the King, no less) and brutal execution of Guy Fawkes. Now, contrary to what I thought, he wasn't burned, but was sentenced to be hung, drawn and quartered – and in my book you can't get much more brutal than "That you be drawn on a hurdle to the place of execution where you shall be hanged by the neck and being alive cut down, your privy members shall be cut off and your bowels taken out and burned before you, your head severed from your body and your body divided into four quarters to be disposed of at the King’s pleasure". Fawkes apparently avoided the latter discomforts by jumping off the scaffold to ensure he broke his neck – his co-conspirators, alas, didn't have the same presence of mind. Given this downright ghoulish story, and the fact that every year, someone gets injured as a result of the fireworks that we use to celebrate the unfortunate Mr Fawkes' demise, I have to ask "why do we do this"? Well, you've gotta admit, those fireworks are mighty impressive….

And they are impressive, at least in part, because of their wonderful colours, which arise as a result of the behaviour of electrons within atoms. Each of the 117 different types of atom currently known has its own unique arrangement of electrons. Consider, for example, the hydrogen atom, which comprises a positively charged nucleus and a single electron. If we consider the nucleus to be stationary and took a large number of snapshots of the atom at various times, we would find that the electron would not always be in the same position. In fact, it would eventually map out a spherical distribution, with its most probable distance from the nucleus being about 0.00000005 millimetres. This is the lowest possible energy state of the hydrogen atom, and we say that the atom is in its ground state.

If we now take this ground state atom and give it a precise amount of energy, we promote it to an excited state. If we were to repeat the above experiment on the excited state atom, the distribution of the electron would now look somewhat like a dumbbell, and the most probable distance of the electron from the nucleus will have increased slightly. The process of promoting an atom from its ground state to an excited state takes place as the result of an electronic transition. There are a large number of possible electronic transitions and hence excited states available to any given atom. Most importantly, however, each type of atom has its own unique set of electronic transitions.

Once the atom has absorbed energy, it must somehow get rid of it, and there are a number of ways it can do this. One of these is an electronic transition from an excited state to one lower in energy, which results in the emission of the excess energy as visible light. There are four such transitions available to the hydrogen atom, and these result in red, blue-green, blue and violet colours. And it is this type of process that gives rise to the beautiful colours we see in fireworks displays. The atoms most commonly used are copper (blue), sodium (orange-yellow), lithium (red), strontium (red), calcium (orange-red) and barium (green). The characteristic colours emitted by these excited state atoms arise as a result of their unique electron configurations, and indeed the colours can be used as an analytical tool to positively identify not only these, but in fact any of the 117 known atoms.

So next time you're at a fireworks display, marvel at the way in which all those electrons are performing for your benefit.