Seven Primitive Unit Cells Video Lecture - NEET

let's now take a look at some of the
other structures of unit cells that we
can have in a dense packing of atoms
done the city dense packing but any sort
of packing of atoms in crystalline
structures so we already saw quite a few
videos on the simple cubic structure
where the dimensions that all directions
are the same size a B and C are the same
and the relative angles between the
various axes if you want to call that
between the sides of the cube they're
all the same so it's a straightforward
simple cube and you know we have
different dimensions depending upon how
atoms are packed in there but as long as
a B and C say the same and the angles
are 90 degrees we have a simple cubic
structure and that's a very common unit
cell structure in chemistry but we also
have other ways in which crystalline
structures will will be packed together
we have what we call Tetragon ultra
trigonal depends upon how you want to
pronounce it where it's basically a
simple cubic structure that has been
elongated so a and B are still the same
the basis is the same that it's now
instead of a straightforward cube it's
now rectangular cube so to speak so it's
a longer tube in one dimension so a is
equal to B but not equal to C see
typically longer shorter and then Abe
well I would say typically longer and
the angles are still the same so still
90-degree angles in all directions so
now we have ortho rhombic which is
basically the same as tetragonal except
now a and B are no longer the same a is
not equal to B and it's not equal to C
so therefore we now have a basic cubic
structure that where the size ABC are
not equal to each other we still have
the same angles 90-degree angles so it's
kind of like a box a shoebox so to speak
where the dimensions in in all three
directions in all three directions or
dimensions are not the same anymore so
the next shape rhombohedral you can see
that there's still a fair amount of
symmetry to have a b and c is the same
so the sides are still the same it's
basically taking what we have a cubic
structure here that's the same and it's
pushed over in such a way that the angle
is no longer you'll to 90 degrees now
the question is which angle well all
three angles are no longer 90 degrees
but the angles are still the same to
each other with other words what's
happened is it's being squished over in
one direction the second direction and
third direction such a way
that the angle alpha angle is the same
as the beta angle is the same as the as
the gamma angle alpha beta and gamma is
all the same and so what basically is
being squished in this direction
squished in that direction squished in
the last direction so in all three
directions is squished over the same
amount so that's kind of a squeeze
basically if you look at it every single
side now will be kind of like a diamond
shape instead of being a perfect square
shaped will now be a diamond shape this
will be diamond shaped I'll be damage
shape so every basic shape every side of
that from rhombohedral is now a diamond
shape okay the the now four five now the
fifth one here we talk about monoclinic
so monoclinic and rhombohedral are kind
of similar the only difference is that a
B and C are not the same in size they're
all different in size so it's kind of
like an orthorhombic shape or a b and c
are not the same but it's being pushed
over in one direction monoclinic means
that it's leans over or push over in one
direction only such way that alpha and
gamma are equal to 90 degrees but betas
now so basically in this in this view
from here it's been pushed over to the
right in one direction only triclinic
here is a very similar to rhombohedral
the only difference here is that a b and
c are not the same so the various
dimensions depth width and height are
not the same and triclinic mean that
it's leaning in three directions but the
lien is different in every dimension
every direction here the lien was the
same in every direction here's the lien
is different in every direction so
that's why the angles are not equal to
each other and none of them are equal to
90 degrees so it's basically leaning in
the three dimensions and finally the
seven shape is what we call hexagonal so
it's kind of like the benzene ring at
the top there's still an atom in the
middle like that that there's an
octagonal shape and then of course it's
also structured in the length direction
like this notice that a and B are all
the same so in this respect we don't
have a square at the top but every one
of these sides are equal in length we
can also notice that these lengths are
not the same as these lengths right here
so the C direction is different from
these directions right here notice that
alpha and beta is 90 degrees so it's
standing
right up is not leaning and the gammas
honor 20 degrees which means that the
internal angles let me show you so this
internal angle right here would be 120
degree angle which is normal for in
hexagon so it's a straight it's like a
hexagon standing straight ups not
leaning and these sites are all the same
but it's higher this way down lengths
down lengthwise along the sides so those
are 7 basic structures basic crystal
structures we will see more that later
on as long as right now at this point
you just realize there's various ways in
which crystalline structures can be
organized and so there's a whole bunch
of crystals out there that are formed
that are in these particular shapes
depending upon the molecular forces
between the molecules and the ions and
so forth that come that come together
and form crystals depending upon how
those are structured we end up with
these 7 basic shapes the one that we're
most familiar with and we'll use most of
the time will be this simple cubic
structure which is the time the
structure you will see in most kind of
compounds that we deal with in organic
chemistry