Crystalline structure demonstrated using a bubble raft. The bubble raft (or Bragg Raft) consists of a 2-dimensional array of bubbles of uniform size. These bubbles represent the atoms that make up a crystalline array. These bubbles have naturally formed a hexagonal packed structure, which is one of the natural types of crystalline structure. The bubble raft also demonstrates structural defects, ranging from holes (vacancy defects) to extended fracture planes (one across centre). There are also dislocational and substitional defects. All these defects are found in crystalline materials such as metals, alloys and semiconductors.