Understanding the Sodium Chloride Crystal Structure

The question asks about the arrangement of ions in a crystal lattice of sodium chloride (NaCl). Specifically, it wants to know how many negatively charged chloride ions (anions) are immediately surrounding a positively charged sodium ion (cation). This number is known as the coordination number of the sodium ion in the NaCl structure.

The structure of solid sodium chloride is a well-known example of an ionic crystal lattice. It can be visualized in a couple of ways:

• Imagine a face-centered cubic (FCC) lattice of chloride ions, with the sodium ions occupying all the octahedral holes within this lattice.
• Alternatively, imagine an FCC lattice of sodium ions, with the chloride ions occupying all the octahedral holes within this lattice.
• Another way to think of it is two interpenetrating FCC lattices, one of sodium ions and one of chloride ions, offset by half the unit cell edge length.

Coordination Environment of Sodium Ions

Let's focus on a single sodium ion within the crystal lattice. Due to the electrostatic attraction between the positive sodium ion ($\text{Na}^+$) and the negative chloride ion ($\text{Cl}^-$), each sodium ion will be surrounded by as many chloride ions as geometrically possible while maintaining stability.

In the NaCl structure, each sodium ion ($\text{Na}^+$) is located at a position where it is equidistant from six chloride ions ($\text{Cl}^-$). These six chloride ions are located at the corners of an imaginary octahedron centered around the sodium ion.

Similarly, each chloride ion ($\text{Cl}^-$) is also surrounded by six sodium ions ($\text{Na}^+$), also arranged octahedrally.

Therefore, the coordination number for both the sodium ion and the chloride ion in the sodium chloride crystal lattice is 6.

Thus, a sodium ion is surrounded by six anions (chloride ions).

Revision Table: Key Aspects of NaCl Structure

Additional Information on Ionic Crystal Structures and Coordination Number

The coordination number in ionic crystals depends on the relative sizes of the cation and anion, and the stoichiometry of the compound. Different crystal structures result in different coordination numbers. For example:

• CsCl Structure: In the cesium chloride structure, each $\text{Cs}^+$ ion is surrounded by 8 $\text{Cl}^-$ ions (and vice-versa). The coordination number is 8, and the structure can be described as a simple cubic lattice with one ion type at the corners and the other at the body center.
• ZnS (Zincblende) Structure: In the zinc sulfide structure, each $\text{Zn}^{2+}$ ion is surrounded by 4 $\text{S}^{2-}$ ions (and vice-versa). The coordination number is 4, and the structure is based on an FCC lattice with one ion type occupying tetrahedral holes.

The sodium chloride structure (NaCl structure) is common for many ionic compounds where the ratio of cation radius to anion radius is within a specific range that favors octahedral coordination.