What properties make it impossible to oxidize gold?

Gold - the mysterious element

Gold has always been something special. Our ancestors used it to decorate their sanctuaries and palaces, honor their rulers and give dead princes golden gifts in their graves. Gold was a status symbol, a sacred element and an object of value at the same time. No wonder that people have been looking for this golden treasure of the earth from the beginning of history - and even tried to make it themselves.

For centuries, alchemists tried to make gold from base metals - but in vain. © Historisch

In ancient times and in the Middle Ages it was the great goal of many alchemists: Chrysopoeia. Through this transformation process they hoped to produce gold from base materials - the most precious and imperishable of all metals. Instead of laboriously digging for it, the gold could then simply be produced in the laboratory.

Transmutation and the Philosopher's Stone

At that time, the best way to achieve this transmutation was the Philosopher's Stone - a mysterious elixir or process that was supposed to enable normal metals to be “refined” into gold. “Gold was the perfect ripe fruit, so to speak, that the underground deposits of the base metals would mature into if left in the ground long enough,” explains William Newman of Indiana University. The Philosopher's Stone should anticipate and hasten this ripening.

On the basis of the knowledge at the time, the transmutation also seemed by no means unattainable. "Most medieval alchemists thought metals were compounds of sulfur and mercury and perhaps a salt," Newman explains. The path from lead to gold therefore seemed to be feasible through chemical reactions. But despite centuries of experiments, the alchemists were unsuccessful - they neither found the philosopher's stone nor did they achieve the longed-for chryopoeia.

The type of atom depends on the number of protons in the nucleus, with gold it is 79. This cannot be changed with purely chemical methods. © Antoine2K / iStock

The protons are crucial

Today it is clear why this could not work: Chemical reactions can change the bond state of an element, but not its atomic nature. In other words: Lead can be combined with other substances in the most imaginative way - the lead atoms will always remain lead atoms. The properties of an atom are mainly determined by the number of protons in the nucleus.

This atomic number determines its position in the periodic table of the elements and also reveals how many electrons are circling in the shell of the atom - a characteristic that is decisive for the bonding properties. In order to turn lead with the atomic number 82 into gold, the alchemists would have had to take three protons away from the lead. But this does not work with purely chemical means, as we know today.

Gold from the particle cannon

Such a transmutation is by no means impossible. Because in the physical way what alchemists tried in vain is now possible. For this, however, today's alchemists need more than just a Bunsen burner and mysterious elixirs: The modern “Philosopher's Stone” are particle cannons and huge accelerator rings - and enormous amounts of energy.

A look inside a particle accelerator at GSI - such ion cannons are the tools of the modern "gold makers". © A. Zschau / GSI Helmholtzzentrum für Schwerionenforschung GmbH

As early as the 1980s, US researchers at the Lawrence Berkeley National Laboratory (LBNL) succeeded in transmutating bismuth, the neighboring element of lead, to gold. To do this, they shot neon and carbon nuclei, accelerated almost to the speed of light, onto a thin bismuth sheet. During this collision, some bismuth atoms are hit in such a way that six to 15 protons are torn from their nucleus. This creates various radioactive gold isotopes and also the stable gold-197 - the isotope that gold jewelry is made of.

"We could have used lead back then, but bismuth was cheaper because it only has one isotope," explains David Morrissey from the LBNL. This made it easier to separate the starting element from the "transmuted" atoms in the collision debris.

Two million gold atoms per second

Another modern "alchemist's workshop" is in Darmstadt. In the particle accelerators of the GSI Helmholtz Center for Heavy Ion Research there, gold is also produced from “base”, heavier elements. The high-energy particle beam can even produce two million new gold nuclei every second.

That sounds like a lot, but nobody gets rich with it. Because to produce a single gram of gold in this way, the accelerators would have to work for 50 million years. In addition, the particle accelerators require gigantic amounts of energy to produce only this tiny amount of gold. Unfortunately, modern alchemy is therefore not lucrative.

Nadja Podbregar
Status: April 27, 2018