The Structure of Crystalline Solids

Question 1

crystallinity effects material properties like

Answer 1

density
melting point
hardness
transparency

Question 2

atomic solids

Answer 2

atoms at the lattice points
graphite

Question 3

ionic crystals

Answer 3

alternating + and - charged ions in a lattice
NaCl

Question 4

molecular solids

Answer 4

molecules at lattice points held together by VDW forces
ice or quartz

Question 5

crystalline structures

Answer 5

regular arrangement in space of atoms in a solid that is most energetically favorable for a given chemical composition and temperature

several structures are possible depending on the size, charge of the atoms/ions/molecules that make up the substance

Question 6

making a crystal

Answer 6

intrinsic relationship between atomic centers in a metallic crystalline array are not always uniform in all direction

characterized by distances a, b, c, and angles a, b, y

Question 7

unit cell

Answer 7

the smallest group of atoms of a substance that has the overall symmetry of a crystal of that substance, and from which the entire lattice can be built up by repetition in three dimensions

a crystal or grain is made up of many of these repeating units

Question 8

crystalline systems

Answer 8

cubic: halite, galena
tetragonal: zircon, chalcopyrite
orthorhombic: sulfur, staurolite
monoclinic: mica, gypsum
triclinic: feldspar
hexagonal: quartz, calcite
rhombohedral: dolomite

Question 9

diamond structure

Answer 9

octahedral
does not conduct electricity
colorless/transparent
covalent bonds

Question 10

graphite structure

Answer 10

hexagonal
good conductor of electricity
black/opaque
covalent w/ hexagonal sheet otherwise, van der waals

Question 11

dislocations

Answer 11

flaws, or imperfections in the crystal structure

Question 12

nucleation

Answer 12

beginning of phase transformation
characterized by regions in molten metal where tiny nuclei form - the beginning of crystal growth

can result in dendritic crystals on outer region that have different structures than the interior

example: making rock candy from sugar water

Question 13

x-ray diffraction: The Bragg Angle

Answer 13

since crystalline structures are uniform, light interacts with the compound in expected and predictable ways
a characteristic spectrum used to identify different compounds

Question 14

cooling during casting

Answer 14

metal crystals nucleate first at coolest points in the melt
may result in dendritic chill forming on outside of cast, with different crystal structure than interior

growing crystals would be relatively round if left to own devices, in order to minimize the high energy surface of the grain

Question 15

amorphous structure

Answer 15

random distribution of atoms
no long-range 3D order
examples: polymers, glasses, ceramics