its professor Dave, let's talk about molecular geometry
as we begin to learn more about molecules it's important to understand the way molecules are
arranged in three-dimensional space because this will affect how the
molecule does chemistry. a model we can use to analyze molecular geometry is
called the VSEPR model which stands for valence shell electron pair repulsion
this is how we will predict the shape of a molecule. atoms are surrounded by
clouds of negatively charged electrons and when you have atoms in a molecule
together, these electron clouds repel each other. because of this, a molecule
will automatically adopt a particular geometry so as to allow all the atoms to
be as far away from each other as possible
think of these electron clouds as magnets of like charge. the closer you
push them together the more potential energy they have. things want to be at
the lowest energy possible so if you let go they will push apart, lowering their
energies. atoms will do the same thing. take carbon dioxide for example. the
carbon atom has two electron domains, or areas of electron density extending from
it. in order to participate in these bonds the carbon takes an s orbital and
a p orbital and hybridizes them forming sp hybridized molecular orbitals. for now
we can just count the number of electrons domains and use that many
atomic orbitals to describe the hybridization of the central atom in a
molecule. for each energy level there is one s, three p's, and five d's so here we
just need one s and one p for these two electron domains. anything that is sp
hybridized is going to show linear electron domain geometry because the
farthest these two oxygen atoms can be from each other while still being bound
to carbon is this shape which involves a 180 degree bond angle. look at a molecule
like BF3. boron has three valence electrons so it can make three bonds
that means that there are three electron domains surrounding the boron atom. that
makes the boron s, p, p
sp2 hybridized. anything that is sp2 hybridized will exhibit trigonal planar
electron domain geometry. this is the furthest the 3 fluorines can be from each
other while connected to the boron. trigonal planar molecules have
120 degree bond angles. once we get to four electron domains around a
central atom we will need to utilize the third dimension. the carbon in methane
is sp3 hybridized so it has tetrahedral electron domain geometry. these
109.5 degree bond angles put the hydrogens as far away from each other as they can
be, making a shape that would have four sides if we connected the points
atoms with five electron domains are sp3d hybridized and have trigonal bipyramidal
electron domain geometry. basically two pyramids connected at the
base. these complexes have both 90 and 120 degree bond
angles. and atoms with six electron domains are sp3d2 hybridized and have
octahedral geometry resembling an eight sided figure. all the bond angles here are
90 degrees. so in order to figure out the electron domain geometry of a
molecule you just count up the electron domains. the number will tell you the
hybridization and therefore the geometry. besides covalent bonds to other atoms
lone pairs also count as electron domains. take ammonia for example. the
three hydrogens and one lone pair make nitrogen sp3 hybridized so it has
tetrahedral electron domain geometry, but the lone pair doesn't take up as much
space as a bond to another atom so it has a slightly different shape from
methane, and we assign it a different molecular geometry. molecules that are
sp3 hybridized but have one lone pair are said to have trigonal pyramidal
molecular geometry. the oxygen atom in a water molecule is also sp3 hybridized
because it makes two bonds and has two lone pairs for a total of four electron
domains, but the two lone pairs mean this molecule has a bent molecular geometry
it is very important to understand how molecules like carbon dioxide and water
have completely different shapes even though they contain the same number of
atoms. the lone pairs on oxygen are pushing away the electron clouds on the
hydrogen just like an atom would which is why it has tetrahedral electron
domain geometry but the shape or the molecular geometry is bent because the
lone pairs don't take up as much space as a bond to another atom. in CO2, carbon
doesn't have any lone pairs, just bonds to oxygen. so to summarize, the number of
electron domains surrounding an atom, be they covalent bonds or lone pairs
determines the hybridization of the central atom. the hybridization should
contain as many letters as there are electron domains. the hybridization
correlates with a particular electron domain geometry, and within each electron
domain geometry there can be multiple molecular geometries as we replace bonds
with lone pairs
when asked to assign these geometries always start by drawing the correct
lewis dot structure, and then just count up the electron domains let's check comprehension
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