The medieval alchemists thought that it was possible to transform other metals into gold. Well, I suppose it is, but not in the way that they thought. All the methods they knew about are really types of chemistry. Chemistry is basically rearranging or exchanging the atoms within molecules: the atoms remain the same.
Gold is an element, that is, it’s composed of one specific type of atom, in this case with 79 protons and 118 neutrons, and chemistry can never change that. There are 92 different elements found in nature, and it’s the number of protons that makes them different. Lead, for example, has 82; aluminium has 13.
Once scientists understood that atoms are never changed in chemistry, they assumed that was that, and that no element could ever be transformed into another. However, by the 1930s, experiments were being performed to try to add extra protons into uranium, which already has the highest number, 92. At first, they thought they’d been successful, because the results were substances that weren’t uranium, but the real explanation was provided by Lise Meitner and her nephew Otto Frisch in 1939: the uranium was breaking up into fragments, lighter elements with a share of the uranium’s protons (and neutrons). Barium (56) and krypton (36), for example. That’s the process of nuclear fission, which powers atomic bombs and Homer’s nuclear plant.
But later experiments did manage to increase the number of protons without smashing the uranium apart. Neptunium (93) and plutonium (94) and americium (95) were created during the war years. They’re all radioactive, they spontaneously break into lighter elements over time, because large numbers of protons can’t live happily together. In recent times, even higher number elements have been created, but they are even more unstable, usually existing for only tiny fractions of a second. The most recent to be given an official name is copernicium, with 112 protons, but the record, pending naming, is 118. That one breaks up in less than a millisecond.
You probably have one of these artificial elements in your house. Even though it doesn’t exist in nature, it’s possible to make americium in large enough quantities to use industrially. It’s radioactive: the unstable atoms split into two very unequal sized lumps with 2 and 93 protons, with the smaller ones, alpha particles, speeding out into the air. When the alpha particles hit air molecules, they knock electrons off, which temporarily makes the air electrically conductive.
That’s how the device in your house works. A small amount of americium constantly emits alpha particles into a space between electrodes. The collisions with air molecules allow a small electrical current to pass. But if smoke particles, which are gigantically bigger than the air molecules, happen to pass into the space between the electrodes, they get in the way of the alpha particles and absorb them. Electric current is interrupted, and the device makes a loud and annoying beeping sound. Your toast is burning. Or maybe your house.
Now, I don’t know if this is possible, but the element iridium has 2 fewer protons than gold. If you could get it to absorb alpha particles from the americium in your smoke detector, it could be transformed into gold. In only 30 or 40 years, you could have as much as a nanogram.