Crystal Structure Coordination Numbers Explained

The coordination number in a crystal structure refers to the number of nearest neighboring atoms or ions that surround a particular atom or ion in the crystal lattice. It's a fundamental property that helps describe the packing efficiency and symmetry of a crystal.

Analyzing Coordination Numbers of Specific Crystal Structures

Let's determine the coordination number for each of the crystal structures mentioned:

Diamond (A) Crystal Structure

The diamond structure, also known as the zinc blende structure type, is characterized by a tetrahedral arrangement. Each atom (like carbon in diamond) is covalently bonded to four other atoms located at the corners of a tetrahedron. Consequently, the coordination number for the diamond structure is 4.

Sodium Chloride (B) Crystal Structure

The sodium chloride (NaCl) structure is a common arrangement for ionic compounds. In this lattice, each cation (e.g., Na⁺) is surrounded by six anions (e.g., Cl^-) in an octahedral configuration, and similarly, each anion is surrounded by six cations. Therefore, the coordination number in the NaCl structure is 6.

Cesium Chloride (C) Crystal Structure

The cesium chloride (CsCl) structure represents another type of ionic crystal lattice. In this structure, each ion (e.g., Cs⁺) is surrounded by eight oppositely charged ions (e.g., Cl^-) positioned at the corners of a cubic unit cell. Likewise, each anion is surrounded by eight cations. The coordination number for the CsCl structure is 8.

Zinc with Hexagonal Closed Packed (D) Structure

The hexagonal closed-packed (hcp) structure is one of the densest ways to pack identical spheres. In an hcp lattice, each atom has 12 nearest neighbors: 6 atoms in the same basal plane, 3 atoms in the layer above, and 3 atoms in the layer below. Hence, the coordination number for the hcp structure is 12.

Table Summarizing Coordination Numbers

Ordering Crystal Structures by Coordination Number

To arrange the given crystal structures in ascending order based on their coordination numbers, we list them from the lowest coordination number to the highest:

• Diamond (A): Coordination Number = 4
• Sodium Chloride (B): Coordination Number = 6
• Cesium Chloride (C): Coordination Number = 8
• Zinc with hcp (D): Coordination Number = 12

Therefore, the correct ascending order of coordination numbers for the crystal structures is A, B, C, D.