What is the primary reason metals exhibit luster?

Metals exhibit luster primarily because they reflect light well. This property arises from the structure of metal atoms and the presence of free electrons, which can move easily throughout the metal lattice. When light strikes the surface of a metal, these free electrons can oscillate and re-emit the light, resulting in a characteristic shine or luster. This reflective ability is a key feature that distinguishes metals from non-metals, which typically do not have the same degree of reflectivity.

In contrast, the other options do not directly contribute to the luster of metals. Low density does not inherently lead to a shiny appearance; rather, density is a measure of mass per unit volume and may vary among different metals. The size of atoms does influence some properties of materials, but it does not play a significant role in reflectivity or luster. Similarly, having a high melting point relates to the thermal stability of a metal but does not impact its ability to reflect light. Thus, the strong reflective quality of metals is the primary reason for their luster.