The reason that mercury (Hg) mixes well with gold (Au) but not at all with copper (Cu) is due to their respective interactions at the atomic level.
Mercury and gold can form an amalgam, which is a homogeneous mixture of two or more metals, in this case, mercury and gold. This ability of mercury to form an amalgam with gold is primarily due to the nature of their atomic interactions and the similarity in their electron configurations.
Gold and mercury both belong to the same group of elements in the periodic table, known as the transition metals. They have similar electronic structures, specifically in their outermost electron shells. Gold has one electron in its outermost 6s orbital, while mercury has two electrons in its outermost 6s orbital. This similarity in electron configuration allows for effective mixing and bonding between the two metals.
On the other hand, copper has a different electron configuration compared to mercury and gold. Copper belongs to a different group in the periodic table, and its outermost electron configuration is 3d¹⁰ 4s¹. This difference in electronic structure makes the interaction between copper and mercury less favorable, resulting in poor mixing and no formation of an amalgam.
In summary, the ability of mercury to mix well with gold and not with copper is primarily due to the similarity in electron configurations between mercury and gold, which allows for effective mixing and bonding, while the different electron configuration of copper leads to poor mixing and no amalgam formation.