12.3.5 Crystal Packing

Question 1

Crystal Packing

Answer 1

• The seven primitive unit cells can be arranged as simple cubic, body-centered cubic, and face-centered cubic.
• The most efficient packing of atoms occurs with hexagonal close-packing and face-centered cubic (also called cubic close-packed).

Question 2

note

Answer 2

• X-ray diffraction is used to determine the position of atoms in a crystal lattice. The seven primitive unit cells are: cubic, tetragonal, orthorhombic, monoclinic, hexagonal, trigonal (also known as rhombohedral), and triclinic.
• Simple cubic unit cells only have atoms in their corner regions.
• Not all crystals only contain atoms in their corner regions. Many crystal systems have atoms packed between the atoms of the primitive structures.
• Placing an atom inside a cubic cell creates a body-centered cubic unit cell (bcc).
• Adding an atom to each of the faces of a cubic cell creates a face-centered cubic unit cell (fcc).
• Hexagonal close-packed structures are composed of layers of hexagonally arranged atoms, each layer being slightly shifted from the one below it in such as way that the atoms from one layer fit into the crevices of the other. This formation aligns the third layer with the first layer.
• The cubic close-packed arrangement is a densely packed arrangement in a crystalline structure in which the fourth hexagonal layer is aligned with the first layer. It is the same as the face-centered cubic structure. This looks like a hexagon from the top. The same hexagonal base is used to create a face-centered cubic lattice (cubic close-packed) but the stacks above it are arranged to align every fourth level.
• The chloride ions in sodium chloride are structured in a face-centered cubic, close-packed arrangement with sodium ions in between. The spaces between the atoms of a face-centered cubic lattice are called octahedral holes (surrounded by ions in the shape of an octahedron).
• Zinc sulfide (ZnS) contained two crystal structures: a
  face-centered cubic lattice with tetrahedral holes in addition to the octahedral holes. The zinc ions shown are in the tetrahedral holes; they are surrounded by ions in the shape of a tetrahedron.
• There are two tetrahedral holes in the face-centered unit cell.

Question 3

In a sodium chloride crystal, the sodium ions occupy ____

Answer 3

octahedral holes between chlorine atoms in the unit cell

Question 4

Each of the cubic unit cells contains a certain number of atoms that is characteristic of that type of cell. Which arrangement of cell names are ordered from least to greatest number of atoms they contain?

Answer 4

simple, body-centered, face-centered

Question 5

In a sodium chloride crystal, the chlorine ions form all of the following types of crystal lattice except

Answer 5

simple cubic

Question 6

In one type of hexagonal close packing, the layers are designated as aba. Using the same notation, the next layer for this type of packing would be designated as

Answer 6

b

Question 7

How many octahedral holes are contained in a face-centered cubic crystal?

Answer 7

the same as the number of atoms or large ions

Question 8

There are how many different types of cubic unit cells?

Answer 8

three

Question 9

What are the two different cells that can be produced by hexagonal close packing?

Answer 9

hexagonal and face-centered cubic

Question 10

X-ray diffraction tells us what about a crystalline lattice?

Answer 10

Where the atoms are

Question 11

In a body-centered cubic lattice, each atom has 8 nearest neighbors. How many nearest neighbors does an atom in a face-centered cubic lattice have?

Answer 11

12

Question 12

The seven types of unit cell discussed are known as what?

Answer 12

Primitive unit cells