 The general symmetry of a layer might also change, as in the case of the Pt (100) surface, which reconstructs from a cubic to a hexagonal structure.A reconstruction can affect one or more layers at the surface, and can either conserve the total number of atoms in a layer (a conservative reconstruction) or have a greater or lesser number than in the bulk (a non-conservative reconstruction).The relaxations and reconstructions considered above would describe the ideal case of "atomically clean" surfaces in vacuum, in which the interaction with another medium is not considered.