MAT E 273 Exam 1 (ISU)

Which type of bonding generally results in positional constraints on atoms due to the shapes of their valence electron orbitals ?

Covalent

Which of the following bonding scenarios features charged atom cores surrounded by free electrons ?

(a) Covalent (b) Ionic (c) Metallic (d) Hydrogen (e) Van der Waals

Metallic

_________ bonding is responsible for binding separate sheets of graphene together to form graphite.

Van der Waals

_________ bonding is responsible for binding separate water molecules together to form a crystal of ice.

Hydrogen

Which of the following systems is not expected to feature bonds between atoms whose valence electron orbitals are sp3 hybridized ?

(a)Silicon

(b) Germanium

(c) Bronze [Cu&Sn alloy]

(d) Carbon [Diamond]

(e) Si-Ge Solid Solution

(c) Bronze [Cu&Sn alloy]

The most commonly produced structural form of metals can be described as:

Random Polycrystal

Identify the valid unit cell that conveys the highest symmetry of the following 2-D lattice

(Hexagon)

What is the coordination number of any aluminum (Al) atom in a defect-free piece of aluminum at a pressure of 1 atmosphere and a temperature of 0oC ?

12 (FCC)

How many total iron (Fe) atoms reside with a unit cell of iron that is at 0oC and 1 atmosphere of pressure ?

2

Which crystal structure(s) feature an atomic packing factor (APF) of 74%.

FCC & HCP

The mobility of ___________ generally dictates the mechanical properties of most metals that are near room temperature and under an ambient pressure of 1 atmosphere.

Dislocations

Compute the theoretical density of Molybdenum (Mo) metal (under STP conditions), given that its atomic radius is 0.1363 nm and its atomic weight is 95.94 g/mol.

10.22 g/cm3

Laboratory furnaces sometimes feature glass windows that allow visual inspection of a sample during a heat treatment. Such windows are often a source of leakage for the gas(es) contained within the furnace. Consider a 3 mm thick sheet of silicate glass that has a cross-sectional area of 600 mm2 that is used for a window. Compute the steady-state flow rate of helium gas (magnitude, in atoms per second) that diffuse through the window if the concentration of helium inside the furnace is maintained at 6x1023 atoms/m3, whereas the helium concentration outside the furnace is maintained at essentially zero atoms/m3. The diffusion coefficient of helium in this silicate glass, at the operating temperature of the furnace, is 1.0x10-10 m2/s.

24x10^12 atoms

A reduced-sphere depiction of the diamond cubic unit cell. The lattice parameter of diamond is 3.567 Å. In terms of fractional lattice vectors, if atom A is positioned at the origin then atom B is displaced by 1/4, 1/4, -1/4 and atom C is displaced by 1, 1, -1. Compute the distance between the centers of the carbon atoms B and C.

4.63 Å

a reduced-sphere depiction of the diamond cubic unit cell. The lattice parameter of diamond is 3.567 Å. Compute the total area of a single {110} plane that intersects with (and is contained within) this unit cell.

17.99 Å2

2-fold symmetry

diamond (diad)

3 fold symmetry

triangle (triad)

4-fold symmetry

square (tetrad)

6 fold symmetry

hexagon (hexad)

Anisotropy

directionally dependent (single crystals)

APF SC

.52

APF BCC

.68

APF FCC

.74

Ionic/Covalent Characteristics

Strong, brittle, hard, stiff

Refractory

Low expansion coefficient

Insulators

Low density

Metallic characteristics

Strong, ductile, tough

Refractory

Conductors

Shiny

High Density

Keesom Forces

dipole-dipole (permanently polarized)

Debye Forces

dipole-induced dipole

Pauling Trends

0

nonpolar covalent electronegativity

Pauling trends

.4

polar covalent electronegativity

pauling trends

1.7 < x

ionic electronegativity

Tetr = Tetragonal (body-centered)

rectangle

Alkai metal halide compounds

NaCl, KBr, CsCl, LiF

VERY ionic

C - Graphite, C60, C-nanotube

sp2 covalent with london between

C-diamond, Ge, Si

sp3 covalent

SiC SiGe

B4C Si3N4

Predominantly covalent

Any n-alkane ever

e.g. methane, propane, polylethylene

covalent bonding within a molecule (C-C, C-H), & London between molecules

Noble gases (neutral, no plasmas) bonding type

100% london forces

How to compute theoretical density?

1. Find mass/unit cell((#atoms/unit cell)/(Avogadro's #))*(Atomic weight)

2. Vc = a^3

eg. (2Rsqrt2)^3

n = #atoms

A = atomic weight

Na = avogadros #

2. Compute density

How to find concentration of vacancies

1. Find atomic density; Nv

2. Use the self diffusivity eqn. (aka Arrhenius Behavior) (w/ activation energy)

No = NaP/A

Nv=Np(exp^[-Ea/kT])

How to find concentration of impurities?

1. Find avg impurities/celln=(%impurities)*(#atoms in cell)

2. Find volume of unit cell

3. n/V

How to find moles of gas that pass through certain area every hour? (diffusion)

1. Find Flux AJ =M/t

2. Flux*Area

How to name directions (4 steps)

1. Place origin at BEGINNING of vector

2. Write vector coordinates

3. Reduce values to lowest integers

4. Enclose in BRACKETS

How to name planes (4 steps)

1. Set to origin not on plane

2. Note where the plane intersects each unit vector (0.5 if 1/2 way and infinity if never)

3. Take reciprocal of these values (reciprocal of 0 is infinity)

4. Enclose in PARENTHESIS

least ordered to most ordered crystal

amorphous, random polycrystal, textured, single crystal

Eutectic

liquid transforms to two solid phases

solvus line

where material precipitates

Closed-Packed Direction (SC)

Closed-Packed Direction (BCC)

Closed-Packed Direction (FCC)