Physical Chemistry Third Edition

28.1 The Structure of Solids 1155

a

Na^1

Cl^2

Figure 28.1 The Sodium Chloride Unit Cell.

Table 28.1 Crystal Systems and Lattices

System Defining Conditions Bravais Lattices

Cubic abcP,I,F

90 ◦αβγ

Tetragonal abcP,I

90 ◦αβγ

Orthorhombic abcP,C,I,F

90 ◦αβγ

Monoclinic abcP,C

90 ◦αγβ

Triclinic abcP

αβγ

Trigonal (rhombohedral) abcP

90 ◦αβγ< 120 ◦

Hexagonal abcP

90 ◦αβ,γ 120 ◦

The hexagonal unit cell can be cut into three parallelepiped unit cells, which are iden-

tical except for their orientations. Either the full hexagonal unit cell or the smaller unit

cell can be used, but the smaller unit cells must be rotated as well as translated to

reproduce the crystal lattice.