In coursework in subjects like
human physiology, we're really very
rarely interested in atoms or an atomic
structure for their own sake. What we're
really interested in more is how
particles interact with each other. How
do atoms interact with other atoms? And
in this sense, we're very interested in
the valence shell of atoms because as I
noted in the last lecture, this outermost
shell occupied by electrons is the
portion of an atom that will interact
with another atom. And the electrons
within the valence shell are what we
call valence electrons. Those are the
electrons occupying that shell. As a very
quick aside to any chemists who might be
watching this, I realize there are some
problems with this particular definition,
but the places where those problems come
up the most like the transition metals
aren't relevant to coursework in human
physiology, at least not in the
introductory coursework. So we're going to
go with this definition as a simplifying
matter.  If we look at several examples
and identify how many valence electrons
they have, we can start with the simplest
atom hydrogen, which has one electron in
the first shell. So the first shell is
the valence shell, and it has one valence
electron. Oxygen, which we've looked at
previously, has eight total electrons -- six
of them in the second shell, making the
second shell the valence shell, and
therefore it has six valence electrons.
Chlorine has a total of 17 electrons --
seven of them are in the third valence
shell which is the valence shell
as the outermost, so they have seven
valence electrons. Atoms with closed
valence shells are the most stable, and
are least likely to react with other atoms.
This idea of achieving stability is
actually really critical here. Atoms that
don't have closed valence shells tend to
interact with other atoms in ways that
lead to closed valence shells and
greater stability. So what determines
whether a shell is closed or not?
Well, it's all about the number of
electrons within the shell. The first
shell is closed when it's occupied by
two electrons, the second shell is closed
when it's occupied by eight, and the
third shell is closed when it's occupied
by eight. So if we look at an example set of
atoms that have closed valence shells,
here we have helium, neon and argon.
Helium has two valence electrons in
shell number one, so it's closed. Neon has
eight valence electrons in shell number
two; it's closed. Argon has eight valence
electrons in the third shell, making it
closed. These three elements come from a
portion of the periodic table referred
to as the inert gases or the noble gases.
There are some other familiar or
potentially familiar elements there, like
xenon and krypton, as well.  In all,
these gases typically are not found
in compounds with other elements. They
have, in their atomic form, a closed
valence shell to begin with, so they're
pretty much non-reactive. By contrast,
let's go back and look at the three
elements we presented before. Hydrogen
has one valence electron;; its valence
shell needs one more to be closed. Oxygen
has six valence electrons; it needs two
more to close that shell out. Chlorine
has seven valence electrons; it's one
electron away from a closed shell. All
three of these elements would be, 
for lack of a better way of saying it,
they're looking for a way to close those
shells. So the question is, how do atoms
achieve closed valence shells? Well,
there're a few options. An atom can give-
up excess valence electrons to another
atom to close out its valence shell. An
atom can take valence electrons away
from another atom to close out its
valence shell. Or the third option is two
atoms could share their valence
electrons with each other so that each
has a closed valence shell part of the
time. These are the three ways that atoms
interact to close their valence shell,
and when they interact, the result is
what's called chemical bonding. And guess
what's next?
