hey guys I wanted to show you some more
stuff on chem doodle 3d today this time
I wanted to show you about some unit
cells that will be adopted by various
ionic compounds and before we look at
some ionic compounds I just wanted to
start with the very simplest thing so I
just loaded polonium this is a single
polonium atom and what we can do here is
build out the cell and see what happens
so let's just do two let's do two cells
and so we can start to see how this is
going to get built out and this is
actually the true unit cell for polonium
this is a this is a simple cubic
structure and what the unit cell would
be is just having these eight atoms at
the corners of the cell and so this is
really the simplest structure that can
be adopted by any metal or anything like
that and in fact this polonium is the
only metal that adopts this this
structure so this is the simplest thing
that we can see we can build this out
even further let's put let's do five of
them and so you can really see how this
is considered a simple cubic structure
there's really no simpler repeating unit
in three dimensions so that's what this
is gonna look like and so now we can
move on to some more complex situations
let's get rid of this so now we're
looking at cesium chloride
we've got cesium ions or Cs+ in purple
we've got chloride ions or Cl- in green
and let's see what happens if we build
this out let's just go to two and what
we end up with here is something that if
these were all the same element this
would be a body centered cubic structure
which is slightly more complex because
we see this simple basically the same
orientation we had before but there's an
additional
atom in the center of that lattice and
so if they were all the same element
we'd be calling it a body centered cubic
structure and a lot of metals do
crystallize it that way but because this
is an ionic compound when we have two
different elements we are not going to
call it a body centered cubic it is
actually still a simple cubic structure
if we consider the cesium ions forming a
simple cubic structure and then the
chloride ions are filling all of the
cubic holes so when when ionic compounds
crystallize they want to make the best
use of the space as possible they want
to fill up all the space so that they
can maximize the ion-ion interactions in
the last and so that's what this is
gonna form and we can even take a look
at this without those bonds if we want
to see this a little more clearly so
here we can see that this is a simple
cubic structure and the interesting
thing is you can consider it a simple
cubic structure with either of the
elements so we can consider the cesium
ions as forming the simple cubic
structure and the chloride ions
occupying the cubic holes or we could we
could flip it and we could say that the
chloride ions are forming a simple cubic
structure and the cesium ions are
filling the cubic holes either way
whichever way you want to look at it
it's the same thing and let's build this
out a little more how about let's do
four and now we can really see quite a
bit of structure here we can rotate this
a number of ways
and that's what we're looking at once
again let's go ahead and remove those
bonds and we can really see this is
still quite a simple structure right
this is about as simple as ionic
compounds can get and of course this
depends greatly on the relative sizes of
the ions here the sizes of the ions are
quite similar but this change is a
little bit when the sizes of the ions
become quite disparate so let's take a
look at another one let's get rid of
this okay we are now looking at zinc
sulfide so the zinc ions are here in
gray and the sulfide ions are in yellow
and so let's let's build this out just a
little bit so we can get a better sense
of this unit cell and this is actually a
face centered cubic structure so that is
where the sulfide ions are forming the
unit cell centered in the faces of the
cube and then the zinc ions are filling
the tetrahedral holes formed by that
form formed by that structure so this is
what this one looks like and we can we
can build that out even a little more
let's just do three and so we can see
that this is a little bit of a different
structure and this is mainly because of
the discrepancy in the sizes of the of
the ions so the larger chloride ions are
going to form that lattice and then the
zinc ions are going to are going to just
fill in the gaps they're gonna fill in
those tetrahedral holes and so this is
typically the case that the anion is
going to form the the unit cell with the
cation filling in the gaps but we can
see the reverse as well so let's take a
look at something else so now we're
looking at calcium fluoride so we've got
Ca 2+ ions in green and then the
F- fluoride ions over here these are the
small
and so now we're looking at something
where we've got a 1 to 2 ratio right
earlier we were looking at one-to-one
ratios of cation to anion now we have
one calcium ion for every two fluoride ions
so we can see the discrepancy the number
there and now we're also looking at
again we're looking at a difference in
the size of the ions so it is actually
the calcium ions the cations this time
that are going to form the face centered
cubic structure while the fluoride ions
are gonna fill in the tetrahedral holes
though in this case they will only fill
half of the tetrahedral holes so let's
go ahead and build this out just a
little bit and we can get a better sense
of this and so this is obviously a more
complex unit cell because we're not
one-to-one right this is not as simple
as cesium chloride where we can just
have repeating cubes right we do have to
allow for the fluoride ions all to be
making contacts with these calcium ions
and we can get rid of these bonds
sometimes it's easier to just remove the
bonds so we can get a better sense of
how this looks so that's this one a
little bit more complex and once again
we saw previously that the anions were
forming the face centered cubic
also known as cubic closest packing so
that's another way of calling it with
the face centered cubic structure and
but so earlier we saw the anions forming
that structure now we're seeing the
cations forming that structure so once
again it just depends on the relative
sizes of the ions in terms of what these
are gonna do let's go ahead and look at
one more so here we have a form of
aluminum oxide called corundum so this
is Al2O3
so it's even more complex in terms of
the ratio between the cation and anion
so we've got Al 3+ in gray and then
O 2- in red and this is going to
form a rhombohedral structure so they're
not all cubic and there's actually
many of these so we won't go through all
of them but I wanted to show you one
weird one and so this is called
rhombohedral we can see that it's not at
all like the the cubic structure that we
looked at and so let's let's build this
out
once again just a little bit let's just
do three of these since it's rather
complex so here's that we've built the
unit cell out quite a bit and we can
look at this from a few different
vantage points we can zoom in if we like
and really get a good look at this thing
we can also once again get rid of those
bonds if we want a little bit of a
clearer picture of the orientation and
so that's just a little bit about these
about these unit cells so we looked at
the the simple cubic cell which you know
just for polonium is pretty much what
exhibits that in terms of a metal and
then certain ions are going to exhibit
that unit cell with other ions filling
in cubic holes we saw what would have
been a body centered cubic cell we saw
face centered as well so that's another
cool feature of chem doodle 3d it's not
just for covalent compounds we can look
at ionic compounds too you can download
any CIF file and just drag and drop
right onto the screen here and and then
you can you can play with the dimensions
of the unit cell you can spin it around
really get a good sense of how this
thing looks in three dimensions there's
a lot of different things to toggle and
you can really just any ionic compound
you can think of you can put it on here
and take a look at what it looks like so
it's a little easier to visualize than
just the two-dimensional representations
in your textbook so I do find this
useful for getting a better sense of
these lattice these lattice structures
and what the geometry is within them so
ahead and take a look at the link in the
description of this clip it's very
affordable and recommend it and
otherwise I'll see you next time