Gold is weird. It's one of the few metals that doesn’t really oxidize. Even silver and copper—from the same column of the periodic table—form weak oxides. Naively, you might expect that gold would tarnish just like silver. Gold also sits right next to platinum, but it has none of that metal’s catalytic properties.

Then came gold nanoparticles that acted like catalysts, and we were confused by their apparent willingness to take part in chemical reactions.

Now, a pair of scientists has explained that gold's inertness isn't inherent to the atom but rather to the surfaces that gold crystals form. Before we get to the results, let’s first take a look at the traditional explanation for gold's inertness and why an inert material that has no catalytic activity suddenly acts as a catalyst when in its nanoparticle form.

Can’t play, won’t try

Atoms are made of a central nucleus surrounded by electrons. The electrons form a structure, pairing up and filling orbitals from lowest to highest energy, with each orbital requiring a different number of pairs.

These orbitals are not like planetary orbits but rather a kind of volume of influence—they’re sometimes round but have other shapes as well. The highest-energy orbitals are generally farther from the central nucleus, exposing them to the rest of the world. An atom’s readiness to react is due to partially filled orbitals at the highest energy, but nothing is ever quite so neat. For very heavy atoms, the order in which orbitals are filled is complicated, and partially filled orbitals can end up closer to the nucleus.

If that happens, those orbitals are shielded from the Universe by the outermost orbitals, which are full of happy electron pairs. This is why, we are told, gold is inert: Its unhappy electrons are shielded by happy electrons.