In chemistry, crystallography, and materials science, the coordination number, also called ligancy, of a central atom in a molecule or crystal is the number of atoms, molecules, or ions bonded to it. The coordination number is a central parameter that determines the structure of coordination compounds/05%3A_Coordination_Chemistry_I_-Structures_and_Isomers/5.03%3A_Coordination_Numbers_and_Structures). It is the number of points of attachment between the ligands and the metal/05%3A_Coordination_Chemistry_I-Structures_and_Isomers/5.03%3A_Coordination_Numbers_and_Structures). The coordination number of an atom in a given molecule or a crystal refers to the total number of atoms, ions, or molecules bonded to the atom in question. The atoms, ions, or molecules that are bonded to the central atom (or molecule/ion) are called ligands. The coordination number of an atom can be calculated by counting the total number of atoms it is bonded to, be it a single bond or a double/triple bond. The most common coordination numbers are 4, 6, and 8/05%3A_Coordination_Chemistry_I-_Structures_and_Isomers/5.03%3A_Coordination_Numbers_and_Structures). The coordination number of an atom can vary depending on the type of complex or crystal it is in. For example, the coordination number of the central cation (Cr3+) in .