structure of metals (part 2) Atomic packing factor Flashcards
What does apf stand for
Atomic packing factor
simple cubic what is a / r
a = 2R or R=0.5a
what is Vc
Volume of unit cell
What is Vs
Volume of atoms in a unit cell
what is the formula of Vc
Vc= a^3
what is the formula Vs
Vs=n x 4/3 pi R^3
what is the Vc for a simple cubic?
Vc= (2R)^3
Vc=8r^3
what is n for a simple cubic
n =(1/8*8) = 1 atoms due to all the atoms being on the edges
Vs simple cubic
Vs= 1*4/3piR^3
=4/3pi R^3
what is the APF of a simple cubic
0.52
Some characteristics of Sc
-they are rare due to low packing density
-closed-packed directions are cube edges
coordination number of SC
6
Give an example of an element that has Sc
Po (polonium)
How to find the a for a BCC
using Pythagoras
(a(root2))^2 + a^2 = (4R)^2
a=4/(root3) R
what is n for BCC
n= 2
because (1/8*8)+1
APF of BCC is
0.68
why is BCC APF smaller the FCC
due to BCC having less neighing atoms then FCC its APF is less the FCCs’
coordinate number of BCC
8
Example of BCC elements
chromium Cr, W tungsten, Fe(Iron),tantalum, Molybdenum.
What is the a of an FCC
using pythagros theorm:
a^2 + a^2 = 4R
a= s(root2)R
what is the n of an FCC
n = 4
(1/88) + (1/26)
what is the FCC of APF
0.74
co ordination number of FCC
12
feature of an FCC
-atoms are located at each of the corners and on the centers of all the faces of a cubic unit cell
- atoms touch each other along face diagonals
