Ch 3 - The Structure of Crystalline Solids

Question 1

What is the correlation between atomic packing and energy?

Answer 1

dense, ordered packed structures tend to have lower energies

Question 2

How are crystalline materials packed?

Answer 2

• atoms pack in periodic, 3D arrays

- typical of metals, many ceramics & some polymers

Question 3

How are noncrystalline materials packed?

Answer 3

• atoms have no periodic packing

- occurs for complex structures & rapid cooling

Question 4

What is another name for noncrystalline?

Answer 4

amorphous

Question 5

What are the reasons for dense packing (4)?

Answer 5

• typically only one element present, so all atomic radii are the same
• metallic bonding is not directional
• nearest neighbor distances tend to be small in order to lower bond energy
• e- cloud shields cores from each other

Question 6

Describe the Simple Cubic structure (SC)

Answer 6

• rare due to low packing density

- close packed directions are cube edges

Question 7

What is the simple cubic’s coordination number?

Answer 7

6

Question 8

Describe the Body Centered Cubic structure (BCC)

Answer 8

• one atom at each corner plus one in the center

- atoms touch each other along cube diagonals

Question 9

What is BCC’s coordination number?

Answer 9

8

Question 10

How many atoms per unit cell in SC?

Answer 10

1

Question 11

How many atoms per unit cell in BCC?

Answer 11

2

Question 12

Describe the Face Centered Cubic structure (FCC)

Answer 12

• one atom on each corner
• one at the center of each face
• atoms touch each other along face diagonals

Question 13

What is FCC’s coordination number?

Answer 13

12

Question 14

How many atoms per unit cell in FCC?

Answer 14

4

Question 15

Describe FCC’s stacking sequence

Answer 15

ABCABCABC….with 2D projection

Question 16

Describe the Hexagonal Close-Packed structure (HCP)

Answer 16

• ABAB stacking sequence with 2D projection

- hexagonal instead of cubic

Question 17

What is the coordination number for HCP?

Answer 17

12

Question 18

How many atoms per unit cell in HCP?

Answer 18

6

Question 19

Describe the tetragonal bravais lattice

Answer 19

• base is a square
• z axis not equal to side lengths
• all internal angles are 90 degrees

Question 20

Describe the rhombohedral (trigonal) bravais lattice

Answer 20

• all sides equal

- all angles equal but not right angles

Question 21

Describe the orthorhombic bravais lattice

Answer 21

• all sides unequal

- all angles equal to 90 degrees

Question 22

Describe the monoclinic bravais lattice

Answer 22

• all sides unequal

- two right angles, third is not

Question 23

Describe the triclinic bravais lattice

Answer 23

• all sides unequal
• all angles unequal
• no right angles

Question 24

What is a unit cell?

Answer 24

smallest repetitive volume which contains the complete lattice pattern of a crystal

Question 25

What is the algorithm to determine crystallographic directions?

Answer 25

(1) position vector to go through origin
(2) read off projections in terms of unit cell dimensions
(3) adjust to smallest integer values
(4) enclose in square brackets w/o commas

Question 26

What is the linear density of atoms?

Answer 26

number of atoms in a given direction over the unit length of that direction vector

Question 27

What is the algorithm for crystallographic planes?

Answer 27

(1) pick origin that does not go through plane
(2) read off intercepts of plane with axes in terms of unit cell dimensions
(3) take reciprocals of intercepts
(4) reduce to smallest integer values
(5) enclose in parentheses w/o commas

Question 28

What is planar density?

Answer 28

number of atoms in a specific plane over the area of said plane