The purpose of this tutorial is to give you
a little more information about how to diagram
an atom and figure how many valence electrons
it has.
Here’s the way we can do this, and this
pretty much mirrors Example 2 in Module 1.
Let’s take a look at Carbon for example
and find it on the Periodic Chart. This is
the same Periodic Chart [on screen] that you
have access to in the Module 1 folder.
You see that Carbon is here and it’s pretty
clear that Carbon has a 6 as the atomic number.
It has an atomic mass of 12. What that tells
us is that it has 6 protons, and then 12 minus
6 neutrons. Then we can figure out the number
of electrons that it has and how those are
arranged as well.
Looking at how we handle that, looking at
Carbon [C] first and say that the atomic number
and number of protons which equals 6. That,
again, is under atomic number here on the
Periodic Chart.
The number of neutrons is given by the mass
number, which when rounded will be 12. It’s
12.01 on the chart. The reason it’s 12 here
is it gets rounded to the nearest whole number.
On the Periodic Chart it’s 12.1 because
[C] Carbon is a mixture of Carbon 12, Carbon
13, and Carbon 14. C12 has about 98.9 percent,
C 13 is about 1.1 percent, and C14 is 0.005
percent which is also the radioactive one.
When you take into account the masses of each
one of these and their natural abundance,
it comes out to a number greater than 12.
We’re only interested in the main isotope
of Carbon, which is Carbon 12. So you round
it to the nearest whole number in these cases.
[Erases that last part]
Moving on, the number of neutrons here will
be 12 minus 6 because 12 is the mass of the
entire atom. You’ve got to remember that
electrons don’t weigh anything; only protons
and neutrons weigh anything. So it’s 12
for the total mass which is protons and neutrons,
and we subtract out the protons and we are
left with the neutrons which is 6 as well.
The number of electrons we’ll have will
equal the number of protons so we can have
electrical neutrality. We’ve got to have
as many electrons which have a negative charge
as protons which have a positive charge. The
neutrons, of course, have no charge, so they’re
not going to affect the charge at all.
The nucleus of the atom, as we said, [drawing]
consists of 6 protons which I’ll show as
+ signs, and 6 neutrons which I’ll represent
with [ 0 ] zeroes. Now we’ve got to figure
out the electron configuration. The way it
works out is we have the “first row” which
has 2 electrons. We’ll have a total of 6
and we’ve used 2, so in the “second row”
we’ll have 4 [drawing]. Customarily we put
the electrons in pairs although it’s not
necessary for you guys.
How did I do this? Take a look at the Periodic
Chart. You see here in the first row we’ve
got two elements. That tells you there’s
will be 2 electrons in the first row. Now,
down here we’ve got [counting] eight elements
so that tells you there is a total possible
of 8 electrons in the second row. In the third
row we’ve got [counting] possibly 8 electrons
as well. After that things get a pretty hairy
but up to that point we’re pretty sure of
what’s going on. You can only have, of course,
as many electrons as you have electrons to
play with. In the case of Carbon you only
have 6 total. We’ve got two in the first
row and four in the second row.
The second row electrons are the valence electrons.
So the total number of valence electrons in
this case will be 4. That means we can easily
tell how many valence electrons there are
just by looking at the group number [periodic
chart]. You can see that Carbon is in Group
IV. The reason we have group numbers is that
it [the group number] matches the number of
valence electrons in a given element. So we
can say [pointing and counting] that’s 1,
2, 3, 4 so we expect to see 4 valence electrons
in Carbon. When we do the electron configuration
it does indeed turn out to be 4 valence electrons.
That’s it for Carbon.
5:57
Let’s look at a couple of the other ones
here. [erases Carbon drawing] Looking at Magnesium
you can see we have [Periodic Chart] a total
of 12 for the atomic number. The atomic mass
is 24, and remember that is protons and neutrons
because electrons don’t weigh anything.
So we’ll be able to figure out the number
of neutrons from that as well. Then we can
go ahead and figure out the electron configuration,
too.
Apply the same principles as before to this
part b [of the question]. The number of protons
would be 12. This is for Magnesium. The number
of neutrons, and remember I said 24.3 which
is a combination of all the different kinds
of magnesium with all the different isotopes
as well call them. So we round to the nearest
whole number which is 24. Then, minus 12 which
comes out to 12. Then, of course, the number
of electrons will equal the number of protons
so electrons will be 12 as well.
[Drawing] We put the nucleus in; that will
have 12 + and 12 of these as well. In the
first row we’ll have 2 electrons. In the
second row we’ll have a total of 12. As
I said earlier, we can have 8 in the second
row. That’s a total of 10 so far. Then we
go ahead into a third row now, and this one
has 2 electrons. So this example has 2 valence
electrons, which is 2 electrons in the outer
shell. We can confirm this by looking at Periodic
Chart where you can see. Again, it’s how
I did the electron configuration diagram we
had 2 in the first row and we have 2 elements
in the first row. We had 8 elements in the
second row and we had 8 electrons in the second
row. Then we have 2 electrons in the third
row and those two were the valence electrons.
So Magnesium comes out to 12, and you’ll
see that in Group 2 which matches the number
of valence electrons we had. That’s how
we can confirm that. Going back [diagram]
the number of valance electrons; VE = 2.
We’ll do one more for Chlorine [ Cl ]. [Erases
previous drawings] This is part c [of the
question]. [Writing] The number of protons
and the number of neutrons and we’ll do
the number of electrons again. This is all
for Chlorine.
Going back to the Periodic Chart you see Chlorine
down here. It’s got an atomic number of
17. You can see that the atomic mass here
is 35.45 and, again, chlorine is mainly a
mixture of chlorine 35 and chlorine 37, about
a 1 to 2 ratio of each of those and it turns
out that the average of all those masses turns
out to be about 35.453 and we go ahead and
round that to 35 [fill is figures on diagram].
So that becomes 35 minus 17 which is going
to be 18 neutrons. The number of electrons
will match the number of protons which is
17.
[Drawing] Doing the diagram as before, there
will be 17 plusses [ 17 + ] and 18 neutrons,
these little circles here. Then we’ll have
2 electrons in the first row. In the second
row [drawing] we’ll have 8 electrons. So
far I’ve used a total of 10 electrons. In
the third row I’ll have a total of 7 [drawing]
which gives us a total of 17 electrons which
is the number we needed. By looking at the
outer row, here the third row, we can see
the number of valence electrons which is 7.
Let’s see how that gels with the Periodic
Chart [on screen]
Chlorine is over here; we have 2 elements
in the first row, so that was 2 electrons,
we have 8 elements in the second row and that
was 8 electrons in our second row, and [counting]
7 which brings us to Chlorine, which you can
see is in Group VII which is the other to
see how many valence electrons there are to
Chlorine. That gives us our total for Chlorine,
the number of valence electrons, and of course
the number of neutrons and protons. [END]
