If you try to draw an electron-dot structure for a metal, you’ll quickly realize that there aren’t enough valence electrons available to form an electron-pair bond between every pair of adjacent atoms. Sodium, for example, which has just one valence electron per atom (3s^1), crystallizes in a body-centered cubic structure in which each Na atom is surrounded by eight nearest neighbors (Section 10.8). Consequently, the valence electrons can’t be localized in a bond between any particular pair of atoms. Instead, they are de-localized and belong to the crystal as a whole.
In the electron-sea model, a metal crystal is viewed as a three-dimensional array of metal cations immersed in a sea of de-localized electrons that are free to move throughout the crystal. The continuum of de-localized, mobile valence electrons acts as an electrostatic glue that holds the metal cations together.