A metal's luster is due to

energy from the nuclei of its atoms being given off as visible light

the free flow of electrons between its atoms

light energy being absorbed and given off again by its valence electrons

valence electrons giving off energy as they collide with one another

bohr atom

The Bohr model of the atom, developed in the early twentieth century, was an attempt to explain observations about the way atoms and electrons absorb, retain and release energy. The model assumed that the atom has a structure similar to that of the solar system with the atomic nucleus at the center of the atom and the atom's electrons move in circular orbits similar to the way planets orbit the sun. The Bohr model represented a great advancement in the understanding of atomic structure and contributed to the development of quantum mechanics.