Coordination Number

Definition

The coordination number refers to the number of nearest neighbor atoms or ions surrounding a central atom or ion in a crystal structure. This concept is crucial for understanding how atoms are arranged in solids and affects various physical properties, including density, stability, and bonding characteristics of the material.

5 Must Know Facts For Your Next Test

1. Coordination numbers can vary widely depending on the type of crystal structure and the sizes of the atoms or ions involved; common values include 4, 6, and 8.
2. In a cubic lattice, such as in sodium chloride (NaCl), each ion has a coordination number of 6, meaning each ion is surrounded by six others.
3. The geometry of the coordination environment (e.g., tetrahedral, octahedral) plays a significant role in determining the physical properties of the material.
4. Higher coordination numbers typically lead to more complex arrangements and can affect the stability and solubility of materials.
5. Coordination numbers are essential for understanding phase transitions in solids, as changes in atomic arrangements can alter their coordination environment.

Review Questions

• How does the coordination number influence the physical properties of a crystal?
  • The coordination number affects how atoms or ions are packed together in a crystal structure, which in turn influences properties such as density, stability, and electrical conductivity. For example, materials with higher coordination numbers may have different bonding characteristics and exhibit varying levels of hardness and thermal conductivity compared to those with lower coordination numbers. Understanding the coordination number is essential for predicting how a material will behave under different conditions.
• Compare and contrast coordination numbers in different crystal structures and their implications on material properties.
  • Different crystal structures exhibit varying coordination numbers that significantly impact their material properties. For instance, in face-centered cubic (FCC) structures like copper, the coordination number is 12, leading to high density and ductility. In contrast, body-centered cubic (BCC) structures like iron have a lower coordination number of 8, resulting in different mechanical properties. These differences help explain why certain materials are preferred for specific applications based on their crystal arrangement and resultant properties.
• Evaluate how changes in coordination number during a phase transition can affect the structural integrity of a solid material.
  • Changes in coordination number during a phase transition can greatly impact the structural integrity of a solid. For instance, if a material transitions from a high-coordination state to a low-coordination state under certain conditions, it may lead to weaker bonding interactions and increased susceptibility to defects. This shift can result in changes to mechanical strength, thermal stability, or even lead to failure mechanisms like fracture. Understanding these transitions is crucial for designing materials that maintain performance under varying environmental conditions.

Related terms

Lattice Structure: The regular, repeating arrangement of atoms or molecules in a crystalline solid.

Crystal System: A classification of crystals based on their symmetry and the lengths and angles between their unit cell parameters.

Ionic Radius: The effective distance from the nucleus of an ion to the outermost electron shell, influencing how ions interact in crystal structures.