In screencast we want to draw the lewis dot structures
of water and ammonia, and use those to understand
something about the molecular structure of those molecules
So let's start with water, and determining the lewis structure
the first step is to count the number of valence electrons
and so in hydrogen, we have only a single valenece electron
in each hydrogen atom, mulitply that by two for the
two hydrogen atoms, for oxygen we have six valence
electrons. And that gives us a total of 8 valence electrons
Alright, we use the lewis dot structure to figure out how
to distribute those electrons, so we start with a central
atom which can never be hydrogen in these diagrams
since hydrogen can only form one bond
Alright, we surround that oxygen atom with hydrogen
atoms. To that central oxygen atom we draw one bond
to each of the species that's involved in this structure
Again, once we draw one bond to hydrogen
that completes it's electronic shell since it
just has an s-orbital and can only accomidate two electrons
We've distributed four of the electrons in these bonds
out of the total of eight, and so we need to distribute four more
and those are distributed in terms of two lone pairs
on the oxygen atom. So this is a simple lewis rendering
of the structure, but of course these different species
occupy three dimensional space, and in general, will be
repelling eachother, the lone pairs and the various bonds
since they all contain electrons, and electrons will
electrostatically repel eachother, and so we know that
this will adopt a strained tetrhedron type structure, where
the lone pairs will occupy two branches of the tetrahedron
and then the the other two will be from hydrogen atoms
that I've tried to show projected into and out of the screen here
And so if you ignore the lone pairs, we see then that a
water molecule adopts a bent type structure
rather than a linear structure due to the replusion
between the different substituents of the oxygen atom
whether they be bonds to hydrogen atoms or the
lone pairs. We can do the same analysis for ammonia
nitrogen has five valence electrons, we then get three
times one for the hydrogen atoms which give us a
total of eight valence electrons, just like in water
in this case though, the electronic structure picture is
a little bit different, and we around the nitrogen
with hydrogen atoms, we again start by drawing the bonds to each
of the components. This saturates all of the hydrogen atoms
they each have two electrons now that are shared with
nitrogen, we've distributed six of the eight electrons
and the last two electrons have to go in the nitrogen
in terms of a lone pair. Again, this kind of planar structure
does not minimize the repulsion between the electrons
that are participating in this bond, and so instead
We adopt a tetrahedral type configuration
Where we have hydrogen atoms that form trigonal pyrimidal type
structure wit hthe nitrogen, and then the lone pair
projecting up from the nitrogen. So we can see how
analysis of these simple molecules in terms of their
lewis dot structure helps us explain why they take the
shapes that they actually take instead of what you might
imagine just from the very simple molecular formula
where you migh expect water would be linear
and that ammonia would be triangluar. In order to
reduce repulsions with their lone pairs, they actually
adopt tetrahedral type geometries and that accounts the shapes that they have