The coinage metals are unusually good conductors of electricity because their electronic configurations are instead of as the periodic table would lead one to expect. In Chapter 8 we commented on the special stability of filled orbitals. This advantage in energy is enough to permit copper to take one of the two electrons in the outer s orbital and use it to fill the last of the d orbitals. This leaves the s orbitals half empty, and facilitates the movement of s electrons through the solid metal. (Because the d orbitals are more deeply buried, electrons in these orbitals are not as free to wander as are s orbital electrons.) The same mobility of s electrons makes Cu, Ag, and Au good reflectors of light (absorption and immediate reemission at the same wavelengths) and conductors of heat. This is why copper and silver are employed as conductors in electronic circuitry, and why artificial satellites are sometimes gold-plated to reflect sunlight and keep the temperature down.

All of the s and d orbitals are filled in Zn, Cd, and Hg, thereby reducing the mobility of electrons from one atom to the next, and lowering the electrical and thermal conductivity of the metals. With all outer electrons paired, interactions between atoms are lessened, which makes the solids soft and low-melting. Mercury, the transition metal of largest atomic size, is a liquid above -390°C, but this is not a fundamental chemical difference because on a warm summer day cesium and gallium are liquids also.