As we learn with electron configurations it gives us a way to give the probable location
of electrons and also gave us some very important information about our core electrons and our
valence electrons, those electrons on the outer shell that are gonna give us the most
information about our reactivity, physical, and chemical properties. If we look at these
valence electrons and we look at the periodic table we're gonna see some striking similarities.
Let's focus on our main group elements and to start let's look at group 1A that's hydrogen,
lithium, sodium, potassium. If we look at the electron configurations for this group
we start with hydrogen with a 1S1, lithium 2S1, sodium 3S1 and potassium 4S1 you'll notice
that the valence electrons the outer electrons are always S1 what's changing is the energy
levels. This gives us some very important information. We know that we are going to
have a spherical outside shell because they're all S. We also know that this orbital is only
half filled, only one electron and not two electrons. The other thing that's important
to understand about group 1A is that it also tells us something about the reactivity--which
ion is actually going to be the most likely ion. One of the things that we haven't talked
about in depth is the octet rule. It's often referred to the octet rule because it looks
at eight electrons but it's better to understand the octet rule from the standpoint of full
shells. Why whenever we get past and have core electrons do we then move on to the valence
shell? That's because those become stabilized and once we get to a stable core then we start
to get the next valence. Well what are the stable core? Well our noble gases, those things
with a full shell at both the S&P and later on when we get further into the periodic table
S, P and D and even further S, P, D, and F. To get a noble type gas configuration for
our group 1A which is S1 we have two possibilities. We could gain seven electrons or we could
lose one, a little easier to lose one than to gain seven. So therefore the most likely
ion for lithium is not lithium-7 but lithium +1. What does that mean for the electron configuration?
If we think about the full electron configuration for lithium 1s2, 2s1 if lithium loses one
electron it'll go back to 1s2. Well what element has an electron configuration of 1s2? Its
helium. This is called an isoelectric electron configuration. Our elements when they create
ions want to have an isoelectric configuration like our noble gases. Let's look at another
group, group 7A these are our halogens: fluorine, chlorine, bromine, and iodine. What does the
valence electrons look like? Well in that case we start with two: 2s2 and 2p5. If we
go down to chlorine 3s2, 3p5. So the commonality here is that we're going to have a filled
s, s2 and five electrons in our p shape. If we think about the isoelectric element if
we added one electron to fluorine we'll have the same electron configuration as neon so
therefore our halogens are much more likely going to gain one electron to become anions
than they would to lose seven electrons and become a cation. You can see here how looking
at the electron configurations in the periodic table allow us to not only see the similarities
in the electron nature of our groups but also predict which elements would prefer to be
cations and which elements would prefer to be anions and not only that but whether they
will have a cat ion of +1,+2, or +3 or anions of -1, -2, -3.