Chem unit 12

hexagonal close-packed (hcp)

crystal lattice in which the layers of atoms or ions in hexagonal unit cells have an ababab stacking patter (as tightly packed as poss)

unit cell

repeating unit of the arrangement of atoms, ions, or molecules in a crystal lattice (minimum repeating pattern)

cubic close-packed (ccp)

a crystal lattice in which the layers of atoms or ions in face-centered cubic unit cells have an abcabc stacking pattern

packing efficiency

the percentage of the total volume of a unit cell occupied by the atoms, ions, or molecules

simple cubic unit cell

an array of atoms or molecules with one particle at the eight corners of a cube (square packing, aaa pattern)

body-centered cubic unit cell

an array of atoms or molecules with one particle at each of the eight corners of a cube and one at the center of the cell (all group 1 metals and many transition metals)

formula for edge length

l = 2r

formula for sc

r = l/2 (1 eq atom)

formula for bcc

r= (l*sqrt 3)/4 (2 eq atoms)

formula for fcc

r = (l*sqrt 2)/4 (4 eq atoms)

coordination number (N)

number of atoms which it has close contact with (hcp: 12, ccp/fcc: 12, bcc: 8, sc: 6)

substitutional alloy

nonhost metal replaces host

interstitial alloy

added element occupies space between atoms of host

low carbon content

0.05-0.19%; malleable/ductile

medium carbon content

0.20-0.49%; high strength

high carbon content

0.5-3.0%; hard but brittle

Ionic crystals

lattice points are cations and anions, electrostatic attraction, hard/brittle/high melting point, poor conductor bc ions are held in place

Covalent crystals/network solids

lattice points are atoms, covalent bonds, hard/high melting point, poor conductor

Molecular crystals

lattice points are molecules, IMF, soft/low melting point, poor conductor bc neutral species

Metallic Crystal

lattice points are metallic bonds, soft to hard/low to high melting point, good conductors bc of mobile e-

Bragg equation

mlambda=2dsintheta