system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are metals like gold, silver or copper not as reactive as sodium or potassium even though all have only one electron in their outer shell?

 I have learnt that since sodium has only one electron in it's outer shell, the atom becomes highly unstable and reactive, and in order to become stable, it often reacts with other elements to form compounds. But, even less reactive and stable metals like gold and silver have only one electron in the outer shell, why aren't they as reactive as sodium or potassium. Why aren't they unstable and react with other elements commonly?
gpt
Reactivity isn't just about how many electrons are in the outer shell, it's also about how tightly bound those electrons are and which shell they're in. The farther out you go, the less energy involved. Gold is a much much larger atom than something like sodium, so the outer electron shell is much farther out, and more sparse, the energy change involved in the outer shell electrons is much lower, and hence the reactivity is much less. 

Filling the 2nd electron in the 1s (first) orbital, like with hydrogen, is very energetic. That's why hydrogen reacts with basically everything. 

Gold is far larger; there are 79 electrons, filling parts of 14 orbitals. Pulling an electron off the 6s orbital (gold) is a lot easier, hence doesn't involve nearly as much energy and isn't so strongly favoured.

Gold is also big enough that the orbital filling order gets weird, it's lower total energy to fill higher orbitals than to try to overstuff some of the lower ones. In addition...gold "should" have a full outer shell 6s orbital if you're just counting electrons but you end up in an lower energy state if one of those outer electrons drops down to complete a lower orbital (5d for gold). Copper and silver are the same way, they have an electron count that corresponds to having a full outer shell but the electrons are reconfigured to a lower energy state within the orbitals that results in the outer shell having just one.