- [Instructor] We are now going
to talk about valence electrons,
and non-valence electrons, which
are known as core electrons
and so one question that you
might have been asking yourself
this whole time that we've been
looking at electron configurations
is, what is the point?
And the point of electron
configurations is,
is they can give us insights
as to how a given atom
or how a given element
is likely to react with other atoms.
And so just to make that point,
or make it a little bit clearer,
let's look at the electron configuration
of an element that we'll
see a lot of in chemistry,
of oxygen.
So oxygen's electron
configuration is what?
Pause this video and see if
you can work through that.
Well, in a neutral oxygen atom,
you have eight protons
and eight electrons,
so first you're gonna fill the one shell,
then you are going to start
filling the second shell,
so you're gonna go 2s2,
so I have four right now,
I have to have four more,
so then you're going to have 2p4.
And then notice, if I add
up all the electrons here,
I have exactly eight electrons.
Now if I'm thinking about
how might oxygen react,
it's interesting to look at
the outer oxygen electrons.
The electrons that are
in the outermost shell.
So the outermost shell is being
described right over here,
this second shell.
So how many electrons are
in the outermost shell?
You have six electrons here.
So oxygen has six valence,
valence electrons.
And how many core electrons does it have?
And the core electrons
generally aren't reactive,
or aren't involved as much in reactions?
It has two core, two core electrons.
Now, why is six valence
electrons interesting?
Well, atoms tend to be more stable
when they have a filled outer shell,
or in most examples, at
least a filled SNP subshells
in their outer shell.
And so in this situation, you say, okay,
oxygen has six valence electrons,
and oftentimes that's drawn
with a Lewis structure,
and it might look something like this,
where oxygen has one, two, three, four,
five, six valence electrons,
and you might say, hey, it would be nice
if oxygen somehow were able
to share, or get ahold of,
two more electrons,
because then that outermost shell
will have a full number
of eight electrons.
The 2s and the 2p would be
filled then, we would have 2p6.
And so you'd say, alright,
well maybe they can grab those
electrons from something else
and that's actually what
oxygen does a lot of,
it grabs electrons from other things.
You can look at something like calcium.
Pause this video, think
about what the electron
configuration of calcium is,
and then think about how
calcium is likely to react
given that atoms tend to be more stable
when they have a full outer shell,
where both their S and P
subshells are completely filled.
Well, calcium's electron configuration,
I could do it in noble gas
notation or configuration,
it'd have the electron
configuration of argon,
and one of the reasons why
the noble gases are so stable
is that they have a completely full shell.
Argon for example has a
completely full first shell,
second shell, and third shell,
and then to build calcium,
will then have two electrons
in that fourth shell,
so it is argon and then 4s2.
So how many valence
electrons does calcium have?
Well, you could see it right over there,
it has two valence electrons.
What about its core electrons?
Well, a neutral calcium atom
would have 20 electrons,
'cause it has 20 protons,
so it would have 18 core electrons.
Electrons that are less likely to react.
And so you can say, what's the easiest way
for calcium to get to a full outer shell?
Well, instead of trying
to gain six electrons,
it might be a lot easier to
just lose these two electrons.
It is actually the case that many times,
calcium will lose electrons,
and become ionized,
will get a positive charge.
So the big picture here is,
one of the values of
electron configuration
is to think about which of your electrons
are most likely to react.
Those are your valence electrons.
In most cases, your valence electrons
are going to be your outermost electrons.
They're going to be the electrons
in that outermost shell.
Generally speaking, if
you're talking about elements
that are in the S block or the P block,
you can think about how many
valence electrons they have
just based on what column they're in.
This column right over here
has one valence electron.
This column over here has
two valence electrons.
This column out here has
three valence electrons,
four valence electrons,
five valence electrons,
six valence electrons, and
seven valence electrons.
The noble gases here,
they are very unreactive,
so one way to think about it
is they are very very very stable,
they have filled their outer shell.
