Hello from Chem Help ASAP! In this video
we will discuss how to quickly determine
the number of valence electrons you have
in a particular element using the
periodic table. Now on the screen we have
a periodic table. This is even an
abbreviated periodic table, and yet there
are still a lot of elements here. Our
task as organic chemists is made simpler
by the fact that we will typically only
encounter about ten elements in
performing organic reactions, and those
elements are as follows... I'll circle them
as we go through them...
Atomic number one and in the first period,
HYDROGEN. In the second period we will
often encounter CARBON, NITROGEN, OXYGEN, and FLUORINE. In the third period,
just below, we have PHOSPHOROUS, SULFUR, and
CHLORINE. In periods 4 and 5 we have
BROMINE and IODINE. So these are the ten
common elements we will encounter. If
you notice these are all considered main
group elements. They're all pulled from
the main groups. 
These groups, which represent the columns
on the periodic table, are labeled with,
in this case, Roman numerals. This is a
Roman numeral one, Roman numeral two... 
That Roman numeral actually represents
how many valence electrons that element
has. So hydrogen is in Group I. It has
one valence electron.
Carbon is in Group IV. It has four
valence electrons. Fluorine, chlorine,
bromine, and iodine are in Group VII.
They have seven valence electrons. So,
this is a really simple way to use a
periodic table to know how many valence
electrons you have. A word of warning! Newer
periodic tables will not be numbered in
this fashion for their groups. They will
be numbered consecutively 1
through 17. So I'm going to the
bottom of the screen...
This will be in Arabic numbers not Roman
numerals. 1, 2, 3... Let's go
across the d-block. Skip those... 13, 14, 15,
16, 17, and 18.
Okay. Well carbon, formerly in Group IV,
clearly, at the bottom of screen, does not
have 14 electrons. What you can do is
just chop off the tens place. So 14. It's
not 14. It's 4. Carbon has 4 electrons.
Fluorine, atomic number 9, doesn't have 17
electrons. It has 7 [valence] electrons. 
Take off the tens place
So we can still use these new group
numbers to determine quickly how many
valence electrons we have. This is a
really easy shortcut. As you work with
elements over time in organic chemistry,
you're not going to need the periodic
table to know your valence electrons.
You're going to simply be so familiar
with them that you'll have them memorized. But,
in the meantime, the periodic table does
offer a lot of help for determining the
number of valence electrons for each of
our commonly encountered elements.
