Because of a special stability in full and half-full orbitals.
Copper, silver and gold have a single outer electron. This seems disadvantageous, energy-wise, until you look at the orbitals the electrons are in. The lone electron is in an S-orbital. This orbital is thus half full (since s-orbitals can contain 2 electrons), whereas all the other orbitals in copper, silver and gold are filled, and hence exceptionally stable. After a full orbital, the next most stable orbital is a half full one.
Take copper, for example. If it followed the arrow diagram, it would have the outer orbitals 4s2 3d9, with a full orbital, and an orbital that is neither full nor half full. Copper actually has 4s1 3d10, a half full orbital and a full one, which provides higher stability to the atoms.
Copper, silver and gold have a single outer electron. This seems disadvantageous, energy-wise, until you look at the orbitals the electrons are in. The lone electron is in an S-orbital. This orbital is thus half full (since s-orbitals can contain 2 electrons), whereas all the other orbitals in copper, silver and gold are filled, and hence exceptionally stable. After a full orbital, the next most stable orbital is a half full one.
Take copper, for example. If it followed the arrow diagram, it would have the outer orbitals 4s2 3d9, with a full orbital, and an orbital that is neither full nor half full. Copper actually has 4s1 3d10, a half full orbital and a full one, which provides higher stability to the atoms.
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Orbital hybridization allows gold, silver, and copper to shift the electrons around into different, non-standard configurations.