- [Narrator] 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 group.
So 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 are
going to ignore groups
three through 12 for this video.
And so we're talking
about the main groups,
the valence electrons
are the electrons in the
outer most shell or the
outer most 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 as one-s-two,
two-s-two,
two-p-six.
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 three-s orbital.
So the full electron
configuration is one-s-two,
two-s-two, two-p-six, and three-s-one.
When I want to figure out
how many valence electrons
sodium has, right, the
number of valence electrons
would be equal to the number of electrons
on the outer most shell,
the outer most energy level.
For sodium, sodium has
the first energy level,
second energy level, and
the third energy level.
The outer most energy
level would of course
be the third energy level.
So if I see how many electrons sodium has
in its outer most energy level, right,
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 wanna 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.
Alright, 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 one-s-two,
two-s-two, two-p-six,
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 three-s-two.
So three-s-two, 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 wanna figure out how
many valence electrons
chlorine has I have to
look for the electrons
in the outer most shell, or
the outer most 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 outer most 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, right?
So one, two, three, four,
five, six, and seven, like that.
And so the reason I
picked sodium and chlorine
is of course because 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, right?
So it has one more proton than electron,
so it gets a plus one charge.
So Na plus 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 one-s-two,
two-s-two, two-p-six.
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 un-reactive,
and that has to do with the fact
that their electron configurations
are full in their outer
most energy level, right?
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 three-p-five,
we would write,
we would write, three-p-six.
And so the electron configuration
for the chloride anion would be one-s-two,
two-s-two, two-p-six,
three-s-two, three-p-six.
We may as well go ahead and hide that.
One-s-two, two-s-two,
two-p-six, three-s-two,
and then three-p-six.
Let's go ahead and draw it, alright?
So we're no longer talking about
a neutral chlorine atom
here, we're talking about
a chloride anion that picked
up one electron, alright?
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, that gives
chlorine a negative charge,
so it is now the chloride anion.
And so you have an ionic
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, right?
So in our video on the periodic table
we talked about elements, right?
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, it's because of this concept
of electron configurations and drawing out
your valence electrons.
And so we could, we could figure out
how many valence electrons
something else has, right?
Let's say we were asked to figure out
how many valence electrons
oxygen has, alright?
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 that's a very
useful thing to think about
that if you wanna find the
number of valence electrons
think about the group number,
four main group elements.
