Coordination Number Video Lecture - Civil Engineering (CE)

in this video we're going to look at
coordination number and how it applies
to covalently bonded molecules
transition metal complexes metallic
solids and ionic compounds in a
covalently bonded molecule the
coordination number is just determined
by counting the number of bonded atoms
so each carbon in this molecule is
bonded to four atoms
that's coordination number for which the
carbons and this one are only bonded to
three atoms coordination number of three
in this one the carbons are bonded to
two atoms coordination number of two and
finally the carbon in this molecule is
only bonded to one oxygen and so it has
a coordination number one notice it
doesn't matter if those bonds of single
double or triple is simply counting the
number of directly bonded atoms we can
also apply this to ions the carbon in
this carbon ion is bonded to three
oxygens so it has a coordination number
of three transition metal cations form
complexes but bonding to groups called
ligands the coordination number the
transition metal of the complex is again
simply obtained by counting the number
of bonded atoms so this vanadium is
bonded to four bro means coordination
number is four this cobalt has a
coordination number of six is bonded to
four nitrogen and two chlorines and this
iron responded to four nitrogens
coordination number for notice of the
four nitrogens are attached together in
a big ring but the coordination number
is four because it's the number of
directly bonded atoms
in the metallic solid there are no clear
bonds between pairs of atoms the metal
atoms are arranged as closely as
possible in layers and then these layers
stack on top of one another in order to
obtain the coordination number we must
count the number of nearest neighbors so
here we have a layer of atoms and our
close-packed item has three below it it
has six around it and it has three on
top of it so the total coordination
number is 12
the way of thinking about the structure
of a solid is to use unit cells unit
cells are the simplest building blocks
which we repeat in three dimensions to
generate the structure of the crystal so
he's a body centered cubic unit cell
it's a cube with an atom on each corner
and an atom at the center and this brick
is going to be repeated in three
dimensions forever and ever and ever to
generate the structure of the metal the
structure of the crystal we want to work
out the coordination number so here's
our unit cell we're going to color one
of our atoms green and watch what
happens to it so at the moment we've got
one cube we place another one to the
next door to it
two more below and we still have space
for more on top so we're going to place
four more on top and that's going to
hide our green atom so we shrink our
atoms and see that the green atom is
surrounded by eight metal atoms so it's
coordination number is eight
in ionic solids there are also no clear
bonds between pairs of atoms now we have
cations and anions packing together as
closely as possible and again we must
look at the structure and count the
number of nearest neighbors so here's
the sodium chloride structure we have
cations in red and gray chloride ions
here's our unit cell and like we did in
the last example that unit cell has got
to reimagine as a Lego brick that can be
packed surrounded in three dimensions in
order to work out the stretch of the
whole solid so in order to work out the
coordination number of our cation we
look at the number of atoms around it
there are six chlorines and so the
coordination number of the cation is six
for the chlorine we have to actually add
another atom from an adjacent unit cell
so for our chlorine we see that there
are six red sodium's around it so the
coordination number of the anion is also
six
so in order to understand the structure
of solids and to count coordination
numbers we actually have to look at the
crystal structures we can look at
physical models of these or we can use
models on our computers and we have
similar available mobile devices and
some that are available for computers so
you can go on and look at some of the
animations that seen in this video in
more detail and you can control them and
you can rotate them and you can look at
the structures in more details so these
are available for computers with full
screens and also the tablet and phone
size screens