Welcome to part 1 of the general
chemistry review for organic chemistry
I'm Melissa Maribel your personal tutor
and let's start with valence electrons
Valence electrons are the electrons on
the outermost shell of an atom that are
either gained or lost during a chemical
reaction these are the valence electrons
you will need to have memorized. First
group has 1 valence electron, second
group has 2 we skip the transition
metals since they varied and now this
group has 3 4 5 6 7 and the noble gases
have 8 valence electrons. We can transfer
electrons which describes an ionic bond
and we can share electrons which
describes a covalent bond which is what
you have normally seen for Lewis
structures. Remember two electrons form
one single bond, four electrons form a
double bond and six electrons form a
triple bond. Say we wanted to draw the
lewis structure for ch4. First we'd
identify how many valence electrons we
have. Carbon has four, hydrogen has 1
multiplied by four since there is a 4
subscript. Add everything together and we
have eight total valence electrons. If
carbon is in your molecule then carbon
is the central atom this is also due to
hydrogen only being able to make one
bond due to it only having one valence
electron. So we'll place carbon in the
center surrounded by its four valence
electrons and place the four hydrogen's
with one valence electron on each
hydrogen. We stated that two valence
electrons form one single bond so this
is our correct lewis structure and you
know you could have just drawn the
single bonds first instead of drawing
each valence electron since the atoms
have to bond regardless for lewis
structures. On to five different Lewis
structures you'll see an organic
chemistry. Here's the first one we'll
identify the total valence electrons in
the structure Carbon has four valence
electrons times 2 so 8, hydrogen has one
valence electron times 6 so 6.
8 plus 6 is 14 valence electrons both
carbons will be our central atoms and
each will be surrounded by three
hydrogen's everything is bonded together
so we'll place our single bonds and count
up all the valence electrons to make
sure we have 14 total. Remember each
single bond counts as two electrons so 2,
4, 6, 8, 10, 12, 14 check.  You also know the
structure is correct because it obeys
the octet rule which states that an atom
wants to have eight valence electrons
and for carbon it must obey the octet
rule. Here's our second structure,
identify the total valence electrons,
Carbon has 4 times 2 so 8, hydrogen has 1
times 4 so 4, 8 plus 4 is 12 valence
electrons. The carbons are our central atoms
surrounded by two hydrogen's on each
side for now we will place single bonds
and if we count up all the valence
electrons we only have 10 but we need 12.
You typically won't see carbon with any
lone pairs you instead will see carbon
with a higher bond meaning a double or
triple bond. Since we only need two more
electrons we will turn this single bond
into a double bond and this obeys the
octet rule since each carbon has four
bonds which is really eight valence
electrons. Here's the third structure,
identify the total valence electrons
carbon has 4 times 2 so 8, hydrogen has 1
times 2 so 2, 8 plus 2 gives us 10
valence electrons. The carbons are
our central atoms surrounded by one
hydrogen on either side. Place single
bonds and we only have six valence
electrons but we need 10 so we need four
more valence electrons so we'll turn the
single bond between the carbons into a
triple bond and now we get 10 valence
electrons and each carbon obeys the
octet rule. Note: carbon prefers to have a
higher bond vs lone pairs but for
nitrogen and oxygen these atoms most
likely will need lone pairs to follow
the octet rule.
Here's the fourth structure,
identify the total valence electrons
carbon has four, hydrogen has one each
and there are five total hydrogens so
five, nitrogen has five valence electrons.
4 plus 5 plus 5 gives us 14
valence electrons.
Our central atoms are never hydrogen
therefore carbon and nitrogen must be
the central atoms. Now the given chemical
formula is telling us how many
hydrogens go on each central atom.
Carbon gets three and nitrogen gets two,
if we count the valence electrons we get
2, 4, 6, 8, 10, 12  but we
need 14 valence electrons and carbon already
has the maximum amount of bonds which
is four so one lone pair goes on the
nitrogen and both the carbon and
nitrogen now follow the octet rule.
Here's our fifth structure, identify the
valence electrons carbon has four, each
hydrogen gets one so we have four
hydrogens so four oxygen has 6, 4 plus 4
plus 6 is 14 valence electrons.
Our central atoms are carbon and oxygen
the chemical formula tells us that there
are three hydrogen surrounding the
carbon atom and only one hydrogen bonded
to the oxygen. Placing our single bonds
we will see that we only have 10 valence
electrons and we need four more so
oxygen needs two lone pairs since carbon
already has the maximum amount of bonds
and this is the correct Lewis structure.
I've linked up part two right over
here which covers polarity and VSEPR
Theory. So make sure to review that
because it will come up again. That's it
for part one and I'll see you in part
two.
