Most of us are familiar with the Bunsen burner from our high schools days. I hope that mention of it brings a flood of pleasant (if sometimes smelly) memories. One of these memories should be the definition of an element, an idea that is central to the science of chemistry: a pure substance that can’t be broken down into simpler substances. Another should be the periodic table of the elements, which I hope most readers recognise as a beautiful organising concept, rather than a befuddling mystery or a massive learning chore.
Robert Bunsen was one of many chemists who contributed to our understanding of the elements that make up our world. He conceived the burner that’s named after him for one purpose: to observe the colours (spectra) emitted by different elements when they’re heated in a colourless flame. Of course, Bunsen burners have since proved useful for many other purposes. This particular burner was used by Sydney dentist Cedric Salmon for making crowns and dentures.
The work of Bunsen and Gustav Kirchhoff with a burner, a prism and a simple optical system was the beginning of spectroscopy, a field which is still increasing chemical understanding today. Using an instrument rather like this one made by Adam Hilger in 1876, they discovered two new elements: caesium (in 1860) and rubidium (in 1861). After inferring the presence of a new element in certain spring water from the observation of distinctive blue lines in its spectrum, Bunsen followed more orthodox procedures of elemental discovery: he distilled tons of the water and isolated a few grams of the metal from the solid residue. Caesium was named for those blue lines; likewise, rubidium was named for the deep red lines in its spectrum.
Bunsen and Kirchhoff investigated most of the ‘alkali metals’ (lithium, sodium, potassium, rubidium, caesium) and the ‘alkaline earths’ (magnesium, calcium, strontium, barium). These elements had been grouped into ‘families’ based on the similarity of their chemical properties. Other similarities between elements had also been noted, but it was not until 1869 that two ex-students of Bunsen, working independently, pulled together a vast array of chemical knowledge to create the periodic table: Dmitri Mendeleev and Lothar Meyer. As we know it today, the periodic table summarises the relationships between elements and places them in atomic number order; at first, they were ordered by atomic weight, which led to some anomalies.
Mendeleev and Meyer were experimentalists who had studied the chemical and physical properties of the sixty known elements. Mendeleev painstakingly wrote these on a series of cards and tried various ways of ordering them. Some people scoffed when he boldly predicted the existence, and properties, of elements where there were gaps in his table; his boldness was vindicated by the discovery of several of them in his lifetime. The table was altered as new elements were discovered and as new information was discovered about the properties of elements, but this unifying concept has proved even more useful than the Bunsen burner.
It took centuries of investigation, hypothesising, explosions and smells, for scientists to abstract these neat ideas from the soup of stuff around us. Thanks to their work, we understand a great deal about the composition of the Earth, the stars, and living things. And chemists today can design complex molecules with desired properties and build them from their elemental building blocks.