Question Bank: Structure

Question 1

State two typical properties of covalently bonded ceramics.

Answer 1

Brittle (low ductility) High strength High stiffness Low electrical conductivity

Question 2

State two typical properties of metallically bonded materials.

Answer 2

High ductility

Good strength

Good stiffness

High electrical conductivity

Question 3

Why do atoms tend to sit at a specified distance from each other?

Answer 3

The atomic separation occurs at the minimum in potential energy where the attractive and repulsive forces are in balance.

Question 4

What feature of the potential energy vs atomic separation curve results in thermal expansion?

Answer 4

Atoms sit in the bottom of the potential energy well. The well is slightly asymmetric so increases in thermal energy increases the mean separation of atoms.

Question 5

Why are metals good conductors of electricity while ceramics usually are not?

Answer 5

The charge must be carried from one electrode to the other. In metals the free electroncs can carry the charge and are highly mobile. In ceramics the electroncs are bound up in covalent bonds and can’t move.

Question 6

What is meant by long-range order in the context of atomic structure?

Answer 6

A distinct periodicity in structure over many atomic distances.

Question 7

What is meant by an amorphous atomic structure?

Answer 7

No long-range order and atoms are essentially randomly located.

Question 8

What is meant by a unit cell?

Answer 8

The unit cell is the smallest piece of crystal that contains all the information needed to build the crystal.

Question 9

What is meant by the atomic packing factor?

Answer 9

The fraction of the unit cell that contains atom.

Question 10

Consider three of the cubic unit cells: simple, FCC, BCC. Rank them in order of decresing atomic packing factor.

Answer 10

FCC > BCC > simple

Question 11

Why do we expect FCC and HCP unit cells to have the same atomic packing factor?

Answer 11

Both structures are made up of the hexagonal 2D lattice. The only difference involves sliding one plane sideways - this difference does not alter the atomic packing factor.

Question 12

How many atoms are there per unit cell in the simple cubic structure?

Answer 12

(1/8) x 8 = 1

1 atom

Question 13

How many atoms are there per unit cell in the BCC structure?

Answer 13

[(1/8) x 8] + 1 = 2

2 atoms

Question 14

How many atoms are there per unit cell in the FCC structure?

Answer 14

[(1/8)*8] + [(1/2)*4] = 3

3 atoms

Question 15

How many atoms are there per unit cell in the HCP structure?

Answer 15

12 corners, each containing a 1/6 of an atom. There are 2 1/2 atoms on the upper and lower faces. There are 3 atoms entirely within the cell. [12*(1/6)] + [2*(1/2)] + 3 = 6

6 atoms

Question 16

What method should be used to find the normal vector to a plane within a unit cell?

Answer 16

1. Determine the planar indices by taking the reciprocal value of axis intercepts.
2. An intercept is infinite if a plane edge runs along an axis.
3. If the plane intercepts the origin, move it and follow the above process.