00:01
An electron on its own could adopt either
spin. And this forms the basis of the Aufbau
principle, and that states that orbitals of
the lowest energy (i.e. those which are closest
to the nucleus) are filled first. So, for
example, if we look at hydrogen, which is
the simplest element, we have a 1s orbital,
which is shown by that straight line there,
and we have a single electron in that orbital.
And so therefore, our electron configuration
is written simply as 1s1. That is to say,
we have the first shell with the principle
quantum number 1. Then we have the s orbital—okay?—so
that gives us a orbital quantum number of
0. And finally, we have a single electron,
which gives us that superscripted 1: the number
of electrons.
If we then look at other elements—so helium,
for example—we can see we have two electrons.
In this case, though, because there is one
shell—only one shell available, the 1s shell—they
have to pair. And so what you see there is
one electron with a magnetic spin number of
1/2 and the other with a magnetic spin number
of –1/2, both coming together, pairing.
The electron configuration for helium, therefore,
is 1s2, as you can see. If we look at group
1 and we look at the lithium atom, we see
that we actually have, now, two principle
quantum numbers. We have the first shell,
the 1 shell, and we also have where n equals
2, or the second shell. This is because, as
I said before, we don't have those orbitals
available in the first shell. It's only when
we get to shell 2 that we have both s orbitals
and p orbitals available to us to populate.
01:58
So in this case, as we are moving across the
periodic table, we have lithium, which possesses
three electrons. The first two go into the
first shell, and the third one exists as an
unpaired electron in the second shell. So
if we consider boron, which is in group 3
of the periodic table, with five electrons,
shell 1 would be filled. Then the 2s orbital
would be filled, so that the orbital with
the next highest energy on the second shell
will be the 2p set. Okay? So this is where
we have those 3p orbitals on the second shell—second
principle quantum number.