So, what does this have to do with anything?
Why do we care about effective nuclear charge, why do we care about shielding?
Well the reason, is we can use this to learn trends in atomic radius,
and then eventually in ionic radius.
How do we define atomic radius?
Well, we know what a radius is.
A radius would be the distance from the nucleus to the outside of the circle.
However, you can't determine where the outside of that circle is, very well.
Take our half the distance between two nuclei.
It's very easy to find nuclei.
We've got means by which we can do that.
So if we look at this nucleus, and
this nucleus, and determine the distance between that nucleus.
Cut it in half, we can assume that, that would be the atomic radius.
If we had a diatomic molecule, like maybe this was Cl2, again,
we would assume that these two are bumping next to each other, and we would define
the atomic radius as one half the distance between those two nuclei.
So, that's how it's defined.
Now let's see how affective nuclear charge and
shielding, define the size of the atom.
Now, before we do that,
I want you to understand that, it is never an issue of crowding.
Where, because you'd have more electrons, you'd need more space for
them, to move about in.
Remember when we talked about the relative size of
the nucleus versus the size of the atom.
And we said to imagine that a marble was suspended in the middle of a big
football stadium, or
soccer stadium, and we had that marble suspended above the 50 yard line.
The rest of that stadium would be the size of the atom.
Electrons are way tinier than the nucleus, and they're moving about in that space, so
the rest of the stadium, it's never going to be a crowding issue.
What defines the size of these atoms, is going to
be the attractive force that the nucleus is, is pulling, on those electrons.
The more those outer electrons are feeling that nuclear charge, the higher that
effective nuclear charge, the tighter it's going to draw those electrons in.
The lower that effective nuclear charge, the bigger the atom's going to be.
So, never think, well I've got more electrons, therefore it has to be bigger.
That's not the argument.
It is the attractive force that those electrons are feeling that nuclear,
nuclear charge, okay, effective nuclear charge.