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.