before we continue we need to understand
that not all molecules are in one plane
as they are written or drawn on paper
pause the video now and draw methane ch4
i expect you may have drawn it like this
but in reality it is a 3d molecule and
should be visualized as such
imagine now the central carbon atom
surrounded by a sphere
we now need to position the four
hydrogen atoms in such a way as to
ensure that all the paired electrons in
the covalent bonds are as far away from
each other as possible
this is known as electron pair repulsion
theory as the electron pairs in both
bonding and lone pairs repel one another
we will end up with a triangular base
pyramid shape at the bottom with a
hydrogen at each corner and a fourth
hydrogen coming out of the top
this shape with four bonding pairs is
known as a tetrahedral molecule
with a bond angle of
109.5 degrees
you will probably have come across the
ammonium ion nh4
pause the video now and try and draw the
shape of this molecule
this nh4 plus still has four bonding
pairs around the central atom
and therefore will be another
tetrahedral shape the same as methane
let us now look at ammonia
nh3 pause the video and draw a dot cross
diagram for this molecule
here is the dot cross diagram
as you can see there are three bonding
pairs and one lone pair
the lone pair electrons will still repel
the bonding electrons in fact the
repulsion of lone pair electrons is
stronger than that of bonding electrons
consequently the molecule will be based
on a similar shape as before
only this time the lone pairs will repel
the covalently bonded hydrogen atoms
even more
creating a pyramidal molecule
with a bond angle reduced
from
109.5 to 107
due to the extra repulsion of these lone
pair electrons
finally let's look at water h2o pause
the video again and draw a dot cross
diagram
here is the drop cross diagram you will
see that there are now two bonding pair
electrons and two lone pair electrons
the lone pair electrons will repel the
bonding electrons even more than before
creating a v-shaped or bent molecule
with a bond angle of 104 degrees
as you can see all three of these
molecules were based on four electron
pairs either bonding pairs or lone pairs
they were therefore all based on the
original tetrahedral molecule however
the bond angle changed due to the elect
extra repulsion
of the lone pair electrons
you