Sodium chloride crystal cubic structure

An ionic compound typically contains a multitude of ions grouped together in a highly ordered three-dimensional array. In sodium chloride, for example, each sodium ion is surrounded by six chloride ions and each chloride ion is surrounded by six sodium ions (Figure 6.11). Overall there is one sodium ion for each chloride ion, but there are no identifiable sodium-chloride pairs. Such an orderly array of ions is known as an ionic crystal. On the atomic level, the crystalline structure of sodium chloride is cubic, which is why macroscopic crystals of table salt are also cubic. Smash a large cubic sodium chloride crystal with a hammer, and what do you get Smaller cubic sodium chloride crystals Similarly, the crystalline structures of other ionic compounds, such as calcium fluoride and aluminum oxide, are a consequence of how the ions pack together. 

The sodium chloride structure. Sodium chloride crystallizes in a face-centered cubic structure (Fig. 4.1a). To visualize the face-centered arrangement, consider only the sodium ions or the chloride ions (this will require extensions of the sketch of the lattice). Eight sodium ions form the comers of a cube and six more are centered on the faces of the cube. The chloride ions are similarly arranged, so that the sodium chloride lattice consists of two interpenetrating face-centered cubic lattices. The coordination number (C.N.) of both ions in the sodium chloride lattice is 6. that is, there are six chloride ions about each sodium ion and six sodium ions about each chloride ion. 

The Sodium Chloride Crystal Structure. Each atom has six nearest neighbors, with octahedral geometry. This arrangement is known as cubic close packed (ccp). 

Figure 7.7 The structure of a sodium chloride crystal is highly ordered. When viewed with a scanning electron microscope, the cubic shape of sodium chloride crystals is visible. 

Table 5.7. Even though you will be able to find out the crystal system, the Bravais lattice, and the number of atoms from this notation, you will not be able to differentiate among different structures with similar notations. For example, cF8 refers to sodium chloride, diamond cubic, and zinc blende structures, which are different from one another.

Sodium chloride crystal. Diagram represents a small fragment of sodium chloride, which forms cubic crystals. Each sodium ion is surrounded by six chloride ions, and each chloride ion is surrounded by six sodium ions. The tiny NaCI crystals show the cubic crystal structure of salt. 

In compound materials - in the ceramic sodium chloride, for instance - there are two (sometimes more) species of atoms, packed together. The crystal structures of such compounds can still be simple. Figure 5.8(a) shows that the ceramics NaCl, KCl and MgO, for example, also form a cubic structure. Naturally, when two species of atoms are not in the ratio 1 1, as in compounds like the nuclear fuel UO2 (a ceramic too) the structure is more complicated (it is shown in Fig. 5.8(b)), although this, too, has a cubic unit cell. 

Taking the ionic radii for Cs+, Cl , Br , and I from Table 42, calculate in cubic centimeters per mole the volumes which the cesium halides would have if they crystallized in the sodium chloride structure, nnd compare with the values plotted in Fig. 57. 

Figure 5.18.1 The NaCl crystal structure consisting of two interpenetrating face-centered cubic lattices. The face-centered cubic arrangement of sodium cations (the smaller spheres) is readily apparent with the larger spheres (representing chloride anions) filling what are known as the octahedral holes of the lattice. Calcium oxide also crystallizes in the sodium chloride structure.

In some crystals such rotation occurs at room temperature. One of the simplest examples is potassium cyanide, KON the structure is of the sodium chloride type (Fig. 127), and this can only mean that the CN ion is rotating it does not necessarily mean that all orientations are equally probable, but it does mean that frequent changes of orientation occur, such that the effective symmetry of the ion is the highest possible in the cubic system neither carbon nor nitrogen atoms occupy specific positions in the structure but are in effect spread over a number of positions. 

Fig. 4.1 Crystal structures of two 1 1 ionic compounds (a) unit cell of sodium chloride, cubic, space group Fm3m (b) unit cell of cesium chloride, cubic, space group Fm3m. [From Ladd, M.F C Structure and Bonding in Solid State Chemistry, Wiley New York, 1979. Reproduced with permission.]...

SODIUM CHLORIDE. NaO. formula weight 58.44, white solid, cubic crystal structure, mp 800.6 C, bp 1,413 C, sp gr 2.165. Commonly called... 

The alkali metal hydrides contain alkali metal cations and H- anions in a face-centered cubic crystal structure like that of sodium chloride (Section 10.9). Alkali metal hydrides are also ionic in the liquid state, as shown by the fact that the molten compounds conduct electricity. 

William B. Pearson (1921-2005) developed a shorthand system for denoting alloy and intermetallic structure types (Pearson, 1967). It is now widely used for ionic and covalent solids, as well. The Pearson symbol consists of a small letter that denotes the crystal system, followed by a capital letter to identify the space lattice. To these a number is added that is equal to the number of atoms in the unit cell. Thus, the Pearson symbol for wurtzite (hexagonal, space group PS mc), which has four atoms in the unit ceU, is hPA. Similarly, the symbol for sodium chloride (cubic, space group Fm3m), with eight atoms in the unit cell, is cF8. 

---