00:01
So you may have heard me
to use the term valence
electrons. What do I mean by that? Well, remember
what I said right at the very beginning of
this course: that when it comes to chemistry,
we're really only interested in electrons.
00:13
If you're interested in protons and neutrons,
then that's nuclear physics. What we're interested in
is electrons, because it's electron movement
which is responsible for the chemistry and
interactions that we observe. And valence
electrons are effectively those electrons
in the outer shell. Those are the electrons
which are furthest away from the nucleus,
which engage in the reactions.
As indicated earlier, if you look at an idealized
drawing (shown on the board here), which considers
electrons as particles, it's only those electrons
which are in the outermost shell (in this
case, we're looking at carbon) that actually
are responsible for the interaction with other
electrons in the outer shells of other atoms.
01:01
So if we consider carbon as an example here,
we see that we have the 1s2 and then the 2s2
and 2p2 is the makeup of that electron configuration.
The remaining electrons—the ones shown in
green in this case, which are closest to the
core—do not react under normal circumstances.
01:27
So if we look, for example, at the core for
phosphorus, which has the chemical symbol
P, we can see that it has the configuration
1s2, 2s2, 2p6, 3s2, and 3p3. The reality is
that only those five electrons in the third
shell react. The 2p6, 2s2, and 1s2 do not
engage in chemical reactions. Yes, there are
a number of things you can do with those,
but principally speaking, when we're talking
about either covalent or ionic bond formation,
we're talking only about those electrons in
the outer shell. And so often, what you will
find, since those… that's all that matters,
is that this extended electron configuration
is abbreviated, shortened, where we use the
electron configuration of the nearest noble
gas (in this case, neon) and just add on the
outer shell electrons to indicate how it might
react.
Okay. In nature, as you'll observe, the lowest
possible energy is always the most desired
state. So if you consider, for example, how
a ball on a table, if pushed ever so slightly,
will fall the distance from the table to the
floor. Items, objects—in this case, electrons—are
always looking at a way of giving up energy,
giving up their potential energy. And the
most stable atomic configurations are where
we have completely filled shells. So if you
recall, helium, neon, argon, krypton, xenon,
and radon, as shown here on the extract of
the periodic table, are all noble gases. They
all have completely filled shells, and therefore,
they're unreactive and are one of the only
things to exist purely in their atomic form
in nature.