Sodium: It is shiny and many of the properties of metal can be related to something called "metallic bonding". An over simplified description of metallic bonding is the "electron-sea model" in which a metal is envisioned as an array of metal cations surrounded by a sea of mobile electrons.
The metallic luster of metals - their shininess, and reflectivity - is due to the absorption and emission of light without changing the wavelength of the photon. Light is absorbed by the electrons, but rather than the energy being converted to heat or to emission of a different wavelength, the energy is emitted at the same wavelength.