Now that we've
classified our elements into groups on the
periodic table, let's see how to determine the
number of valence electrons. And so for this
video, we're only talking about the
valence electrons for elements in the main groups. When we talk about
the main groups, you're using the one
through eight system for classifying groups. So one, two, three, four,
five, six, seven, and eight. So we're going to ignore the
other way to number the groups. And so therefore, we're going to
ignore groups three through 12 for this video. And so if we're talking
about the main groups, the valence electrons
are the electrons in the outermost shell or
the outermost energy level. And so let's see if
we can figure out how many valence
electrons sodium has. So for sodium, if I wanted to
write an electron configuration for sodium-- I
assume you already know how to do these-- so you
would say it is 1S2, 2S2, 2P6. And that takes you all
the way over here to neon. And then that brings
you to the third period or the third energy level. And you have one more
electron to worry about. And so that electron would
go into a 3S orbital. So the full electron
configuration is 1S2, 2S2, 2P6, and 3S1. When I want to figure out
how many valence electrons sodium has, the number
of valence electrons would be equal to the
number of electrons in the outermost shell,
the outermost energy level. For sodium, sodium has the first
energy level, second energy level, and the
third energy level. The outermost
energy level would, of course, the
third energy level. So if I see how many
electrons sodium has in its outermost energy level,
it's only one this time. So that means that sodium
has one valence electron. And that's very
convenient, because sodium is found in group one. And so we can say
that for main groups, if you want to figure out
how many valence electrons you have, it's just equal
to the group number. So the group number
is equal to the number of valence electrons. And so that makes
everything really easy. And so if I wanted to represent
a neutral atom of sodium with its one valence electron,
I could draw sodium here, and I could draw one valence
electron next to sodium like that. All right. Let's go ahead and write
the electron configuration for chlorine next. So here's chlorine over here. And so if I wanted to write
the electron configuration for chlorine, it would be
1s2, 2s2, 2p6, and once again, that takes me all
the way to neon. And so now, I'm over
here in the third energy level, or the third period. I can see that I would
fill 3s2-- so 3s3. And that puts me
into my P orbitals. So how many electrons
are in my P orbitals? One, two, three, four, five-- so
I'm in the third energy level, I'm in P orbitals, and
I have five electrons. And so that would be the
electron configuration for chlorine. If I want to figure out how many
valence electrons chlorine has, I have to look for the electrons
in the outermost shell, or the outermost energy level. So I have, once again, the first
energy level, the second energy level, and the
third energy level. So I want the total
number of electrons in the outermost energy level. So how many electrons are
in the third energy level? Well, there's two and
five, for a total of seven. So chlorine has seven
valence electrons. And once again, that's
very convenient, because chlorine
is in group seven. And so let's go ahead
and draw chlorine with its seven
valence electrons. So here is chlorine. So one, two, three, four, five,
six, and seven, like that-- and so the reason I
picked sodium and chlorine is, of course, because the
sodium and chlorine will react together to form
sodium chloride. And let's analyze what
happens using our electron configurations. And so sodium is going
to lose one electron. So a neutral atom of
sodium has equal numbers of protons and electrons. But if sodium loses its
one valence electron-- so it's going to lose
its one valence electron, and I can show its
one valence electron, actually, is moving over
here to the chlorine. So now, when I
draw sodium, I have to represent it as
an ion, a cation. Sodium used to have
equal numbers of protons and electrons, but it
just lost one electron. Therefore, it's left with an
unbalanced number of protons. So it has one more
proton than electrons. So it's a plus one charge. So Na+ is the sodium cation. The sodium cation is stable. And the reason why has to do
with the resulting electron configuration. So if I look at the resulting
electron configuration-- let me go ahead and
use yellow here-- it would be 1s2, 2s2, 2p6. And so the electron
configuration for the sodium
cation is the same as neon, which is a noble gas. And we know that noble gases
are generally unreactive, and that has to do with the
fact that their electron configurations are full in
their outermost energy level. So the sodium cation is stable,
because it has an electron configuration like
that of a noble gas. So for chlorine, if we think
about how chlorine reacts, chlorine has seven
valence electrons. And let's find it on
our periodic table here. So here is chlorine. Chlorine has seven
valence electrons. If chlorine gets one
more, then chlorine would have an electron
configuration like a noble gas, like that of argon. So chlorine will gain
an electron here. So let's go ahead and write
the new electron configuration. If a neutral atom of chlorine
picks up an electron, well, the electron
would add right in here. So instead of 3p5,
we would write 3P6. And so the electron
configuration for the chloride anion would
be 1s2, 2s2, 2p6, 3s2, 3p6. Let me just go ahead and
highlight that-- 1s2, 2s2, 2p6, 3s2 and then 3p6. Let's go ahead and draw it. So we're no longer talking about
a neutral chlorine atom here. We're talking about
a chloride anion that picked up one electron. So it took that
electron from sodium. So I'm going to show
that electron in red-- has moved over here to
chlorine, like that. And so chlorine
gains an electron. So it used to be
overall neutral. It used to have an equal
number of positive charges and negative charges. But it just added
one more electron. So that gives chlorine
a negative charge. So it's now the chloride anion. And so you have an
anionic bond that forms between the sodium cation
and the chloride anion here. So the attraction of
these opposite charges forms an ionic bond. And so this is an example
of a group one alkali metal reacting with a halogen. So in our video on
the periodic table, we talked about elements. We talked about these
being our alkali metals. And since these alkali
metals are all in group one, they all have one
valence electron. And we talked about
our halogens over here as also being
extremely reactive. And the reason they
are so reactive is if they add
one more electron, they have the electron
configuration of a noble gas. And so drawing the
electron configurations, thinking about valence
electrons and thinking about the resulting
electron configurations allows you to figure out
how these things react. And so that is the
reason why we can say that group one
metals are so reactive, and why we can say that
group seven halogens, or 17, are so reactive. It's because of this concept
of electron configurations and drawing out your
valence electrons. And so we could figure out
how many valence electrons something else has, right? So let's say we were
asked to figure out how many valence
electrons oxygen has. So all we would need to do
is look at the group number, right? So this would be--
oxygen is in group six. And so therefore, oxygen
has six valence electrons. And so if you wanted
to represent oxygen with its six valence
electrons, you could go ahead and draw in six
valence electrons like that. And so it's a very useful thing
to think about that if you want to find the number
of valence electrons, think about the group number
for main group elements.