Welcome to Chem Help ASAP.
Once we recognize that valence electrons are
important, we need to be able to count how
many valence electrons each atom has.
Fortunately, doing this is as easy as looking
at a periodic table.
Our problem is made even simpler by the fact
that organic chemistry...
You typically only encounter ten common elements.
(in the upper left) What are these ten common
elements that we'll encounter alot.
The first is hydrogen.
Moving down to the second row...
Atoms that we will see covalently bound in
our molecules include carbon, nitrogen, oxygen,
and fluorine.
Those are all in the second period or second
row.
Third row elements: phosphorous, sulfur, and
chlorine.
From the fourth and fifth rows, we'll pull
in the two halogens, bromine and iodine.
These are the only ones that we really need
to focus on with regularity.
As it turns out, these are all what we would
call "main group" elements.
What is a group?
Group refers to these column numbers that
we see on the periodic table.
On this particular periodic table that have,
these are written in Roman numerals.
This is group 1.
There's group 2.
We skip some, and we get 3.
It goes all the way to 8.
Fortunately, our group number equals the number
of valence electrons that we have.
Hydrogen is in group 1, therefore hydrogen
has 1 valence electron.
Carbon is in group 4, therefore it has 4 valence
electrons.
Fluorine, chlorine, bromine, and iodine, all
four of our halogens, have...
They're in group 7, therefore they have 7
valence electrons.
One word of warning...
Not all periodic tables are labeled with the
exact same group numbers.
This sounds terrible, but there is a simple
fix for this.
On the periodic table that we have, we only
go 1 through 8.
Often, and I'll show this at the bottom of
the screen, people will start, again, with
1, 2.
Then they will include what we call the "d-block
elements" in their group number counting.
They go 3 (and it goes all the way over here).
It picks up again at 13, 14... and then continues
on...
17 and 18.
Now, if you say, "It's just the group number
as the valence electrons."
Carbon, now, under this system is in group
14.
Carbon only has 6 electrons so it can't have
14 valence electrons.
What we do in this case...
Just chop off the tens place.
In our old system at the top, it was 4.
Now it's 14.
Chop out the tens place, and once again, we're
back to 4 electrons.
It's the same for the halogens.
Instead of 17, it's 7 electrons.
If we can look at a periodic table, we can
very quickly figure out how many valence electrons
a particular neutral atom will have.
