Question

Which of the following crystals have \(6: 6\) coordination? (a) \(\mathrm{MnO}\) (b) \(\mathrm{NH}_{4} \mathrm{I}\) (c) \(\mathrm{ZnS}\) (d) none of these

Short answer

MnO has 6:6 coordination.

Step-by-step solution

Understand 6:6 Coordination

In a crystal structure, a 6:6 coordination refers to an arrangement where each cation is surrounded by six anions and vice versa, often resulting in an octahedral geometry.

Analyze MnO Structure

The crystal structure of MnO is similar to that of NaCl (rock salt structure), where manganese ions are surrounded by six oxide ions and vice versa. This means MnO has a 6:6 coordination.

Analyze NH₄I Structure

NH₄I (ammonium iodide) has an ionic lattice structure but does not exhibit 6:6 coordination. Instead, it has a 6:4 coordination due to the sterics and size of the NH₄⁺ ion.

Analyze ZnS Structure

ZnS (zinc sulfide) usually forms in either the zinc blende or wurtzite structure, both of which exhibit a 4:4 coordination, not 6:6 coordination.

Key concepts

6:6 Coordination

In the realm of crystallography, coordination numbers are crucial for understanding how ions or atoms pack together to form a solid structure. The 6:6 coordination refers to a scenario where each cation is surrounded by six anions, and each anion is surrounded by six cations. This balanced and symmetrical arrangement aids in forming stable crystal structures.

Such coordination often leads to the formation of an octahedral geometry. This means that if we were to visualize this specific arrangement in three-dimensional space, the ions would form a shape similar to two pyramids connected at their bases. This configuration maximizes the attractive forces between oppositely charged ions and minimizes repulsion among like-charged ions, resulting in a stable structure. It is an extremely efficient way for ions to pack densely and create a solid, leading to increased stability of the crystal.

• Each cation has six nearest neighbor anions.
• Each anion has six nearest neighbor cations.
• Results in an octahedral topology when visualized.

Octahedral Geometry

Octahedral geometry is a common shape for the arrangement of atoms surrounding a central atom in a molecule or for ions in a crystal lattice. This arrangement consists of six atoms, ions, or groups symmetrically arranged around a central atom, forming the vertices of an octahedron. An octahedron is a polyhedral shape with eight faces, twelve edges, and six vertices.

This configuration can be commonly found in the mineral world. In ionic compounds, particularly those with 6:6 coordination, octahedral geometry is prevalent. Each central atom or ion is surrounded equidistantly by six others, optimizing the attractive and repulsive forces to stabilize the entire structure.

• Creates a symmetric 3D shape with six groups surrounding a central atom.
• Common in transition metal complexes and ionic compounds.
• Contributes to the physical and chemical properties of the material.

Rock Salt Structure

Rock salt structure, also known as the NaCl structure, is a classic example of 6:6 coordination in ionic crystals. In this structure, each cation is surrounded by six anions and vice versa, forming an octahedral geometry.

The rock salt structure is cubic in nature and is characteristic of several ionic compounds besides NaCl, such as MnO. This type of arrangement not only underscores stability but also is a common blueprint for the study of ionic interactions in crystalline solids. By understanding this structure, one grasps how similar these arrangements are repeated across different types of materials.

• Exemplifies the octahedral arrangement commonly found in ionic compounds.
• Named after its most famous representative, common table salt (NaCl).
• Influences many compounds with similar stoichiometry and size, creating predictable patterns in crystallography.