Together with caesium and gold (both yellow), and osmium (bluish), copper is one of only four elemental metals with a natural color other than gray or silver. Pure copper is orange-red and acquires a reddish tarnish when exposed to air. The characteristic color of copper results from the electronic transitions between the filled 3d and half-empty 4s atomic shells – the energy difference between these shells is such that it corresponds to orange light. The same mechanism accounts for the yellow color of gold and caesium. COUNT 5
QSN1 How many metals have a natural color that isn't gray? ANS1 four QSN2 What color is pure copper? ANS2 orange-red QSN3 What does copper aquire when exposed to air? ANS3 reddish tarnish QSN4 The energy difference between filled 3d and half-empty 4s atomic shells corresponds to what color of light? ANS4 orange light QSN5 What color is the metal caesium? ANS5 yellow