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 thanks for watching, subscribe to my channel for more tutorials, and as always feel free to email me