topic 2🅱️ Flashcards
coordination number
number of atoms or ions immediately surrounding a central atom in a complex or crystal
coordination number of ccp or fcc and hcp structures
12!!
3 above, 3 below, 6 on the same level
bcc coordination number
8
not closed packed
cubic geometry
primitive cubic structure coordination number
6
octahedral geometry.
the atom is part of a few unit cells
what’s an interstitial site
gaps between closed packed structures
atoms,, even when closely packed,, have gaps between them,, they aren’t packed with 100% efficacy
can interstitial sites be occupied by other atoms
yes!!!
most efficient way of packing
being closely packed
reduces the interstitial sites between them.
how can interstitial sites be categorised
categorised by their geometry
2 close packed type of interstitial site
HCP + CCP can have which interstitial sites
- octahedral site
- tetrahedral site
octahedral interstitial site
between 2 triangles above + below.
between 6 atoms
can see all the way through the structure
tetrahedral interstitial site
- between a triangle above + a single atom below
(can’t see all the way through the structure)
between 4 atoms
amount of octahedral sites in a close packed structure issss
‘N’
where N is the amount of atoms in host
number of tetrahedral interstitial sites in a close packed structure
2N
N is the number of atoms in a host
tetrahedral: between 4 atoms
double the amount of octahedral sites
how can we draw a unit cell diagram
- space filling diagram
- ball and stick diagram
- cell projection diagram
what is a cell projection diagram
a 2D diagram of the unit cell,, viewed from above
what is lost when drawing a cell projection diagram
info on the Z axis!! aka how tall the unit cell is + where the diff atoms are placed on the z axis
how can we show where the atoms are in a cell projection diagram
draw the atoms in diff environments :: code them with either 0,1 or 1/2 to show where they’re located
when drawing a cell projection diagram,, what must we do
look at the diff atom environments
code each different environment
give them a number to show where they are on the Z axis
are close packing structures 100% efficient
no!! they have interstitial sites aka gaps between atoms in a close packed structure
how can we measure how efficient a close packed structure is in terms of packing
find the packing efficiency
what is the packing efficiency
the percent of the structure that is filled by the atoms // ions
packing efficiency equation
volume of atoms in a unit cell // total volume of the unit cell
x100
what must we find to figure out the packing efficiency
we must know the volume of the atoms + the volume of the unit cell
how do we figure out the total number of atoms in the unit cell
- find how many atoms there are + if they’re on a vertices or a face
- find their contribution eg: 1/ how many other unit cells they’re present in
- contribution x total amount of them in ONE unit cell
