Welcome to part three of the general
chemistry review for organic chemistry.
I'm Melissa Maribel your personal tutor
and let's go over formal charges.
Formal charges are the charges of each
individual atom within the molecule
all the formal charges add up to the overall
charge of the molecule. Formal charges
are a great way to check if we drew the
most stable lewis structure there are
two different cases case one is a
neutral molecule and case two is a
charged molecule here's a neutral
molecule of CO2 and here's the Lewis
structure. Let's practice finding formal
charges for each atom. Here's our formal charge formula.
we'll apply this formula for the oxygen on the left.
Start with the typical valence electrons
for oxygen which is 6
next are the bonding electrons this
means the electrons that are found in
the bonds that are directly touching
that oxygen so we have one, two bonding
electrons. Now count each individual lone
pair or really each electron on that
oxygen. So we have one, two, three, four
remember order of operations or PEMDAS, we will start with parentheses first and
then subtract so our formal charge is
zero. The oxygen on the right would have the same formal charge since it is
exactly the same as the one on the left.
Let's find carbon's formal charge. Carbon has four valence electrons, the electrons
directly touching carbon are 1, 2, 3 & 4
and there are no individual lone pairs so 4 minus 4 gives us 0. A neutral
molecule prefers to have all formal
charges be 0 if it's possible for the structure. Case 2, we have a charged
molecule here are all the possible structures we have. Let's use formal
charges to help us see which one is the
most stable, we'll find the formal charges
for each atom in the structure. Oxygen has 6 valence electrons there are two
bonding electrons and four individual
lone pairs so we will get 0 for our formal charge.
Carbon has four valence electrons there are four bonding electrons and no
individual lone pairs. Carbon's formal
charge is also zero. Nitrogen has five valence electrons there are two bonding
electrons and for individual lone pairs. Nitrogen's formal charge is negative one.
Note: the most electronegative atoms want to be negative and looking back at our
electronegativity trend oxygen is more electronegative than nitrogen so this
structure is not the most stable since oxygen should be negative here are the
formal charges for the next structure. Once again oxygen prefers to be negative
and this one is out since the formal charge is a plus one this last structure
is the most stable due to the most electronegative atom of oxygen being
negative and you will also see that the central atom prefers to have a formal
charge of zero. Next is resonance when Lewis structures can be drawn multiple
ways this is known as resonance. Here's the first lewis structure for a
carbonate ion. To find the next structure we can move the double bond to a
different oxygen what we are doing is moving the electrons to this oxygen and
our double bond is breaking and reforming to a different oxygen
resonance is represented by a double arrow there is one more resonance
structure so we'll move the electrons to this other oxygen and break the double
bond here and reform it on this oxygen. These are all of our resonance
structures. This movement of electrons that we just saw is called
delocalization. In organic chemistry we're going to get a bit deeper into the
concept of resonance it is going to be described as a structure that has the
delocalized electrons which is referring to the ability of moving electrons. If we
were to combine all the resonance structures it would give us a resonance
hybrid think of it this way every person has different personality traits let's
say and in my case I'm academic, dorky and persistent combine all those traits
together and you get me. Same goes for
our carbonate ion by combining all the resonance structures or personality
traits this gives you the complete lewis structure which is known as the
resonance hybrid. The more resonance structures a molecule has the more
stable it typically is. Now for hybridization let's find the
hybridization of each central atom for this structure looking at this first
carbon we will identify the electron geometry to find the electron geometry
we must find the electron groups of this carbon electron groups are bonds plus
lone pairs. We have one, two, three, four bonds and no lone pairs so we have four
electron groups. Using our table this is tetrahedral and tetrahedral has a
hybridization of sp3 so each carbon has a hybridization of sp3. Let's find the
hybridization of this structure we'll find the electron groups and note a
double or triple bond only counts as one bond whenever we are finding the
electron groups so this carbon has one, two, three electron groups making it
trigonal planar for our electron geometry and our hybridization for each
carbon is then sp2. Let's find the hybridization for each carbon in this
structure we'll find the electron groups first and the triple bond only counts as
one bond so we have one, two electron groups making it linear so our
hybridization for each carbon is sp.
If you missed part 1 or part 2 you can find
that right over here and if you're ready for an introduction to organic chemistry
you can also find that right over here
and remember stay determined you can do this!