The periodic table

The periodic table captures a subtle pattern that runs through the chemical elements, the fundamental building blocks of everything around us: from the aluminium in bike frames to the xenon gas in glowing shop signs.

It is the number of protons in an atom’s nucleus, known as its atomic number, that determines which element it represents. If you have one proton, that is hydrogen. If you have 12 protons, that’s carbon. As you keep adding protons, you find that similar properties recur in every 8th element. Lithium, for example, with an atomic number of 3, is a reactive metal — just like sodium (element number 11) and potassium (number 19).

A good way to think about it is to imagine running your fingers over the keys of a piano. The notes flow out, resounding at a higher and higher pitch as your hand moves to the right. As you hit the eighth note, something beautiful happens – a note hangs in the air that embodies something of the first.

In tribute to this musical analogy, the chemists who first noticed the pattern in the elements more than 150 years ago called it the law of octaves. We now know the reality is more sophisticated. The underlying factor that largely dictates an element’s properties is the number of electrons that orbit in the shell furthest form the nucleus. It is the arrangement of these electron orbits that is reflected in the different blocks of the periodic table.

The table we know today is a descendant of that invented in 1869 by a wild-haired Russian named Dmitri Mendeleev. He wasn’t the first to try arranging the elements in a table according to their properties, but he was the first to recognise that any gaps must correspond to elements not yet discovered. He didn’t get everything right, but enough of his predictions came true to cement his legacy. Joshua Howgego