We can easily classify if a molecule is polar
or not by looking at whether either one of
these 2 criteria is true. Basically, a molecule
is polar if there are different terminal atoms
or it has unsymmetrical lone pairs on the
central atom. If these 2 criteria is not true,
then the molecule is nonpolar. Now, having
different terminal atoms is pretty straight
forward. However, having unsymmetrical lone
pairs on the central atom may not be as clear.
To classify what unsymmetrical lone pair is
all about, let's start by looking at the VSEPR
geometry chart. We have geometries ranging
from 2 to 6 terminal atoms surrounding the
central atom. Since we're talking about lone
pairs, that means, these geometries having
0 lone pair are not concern. So, we're gonna
focus on the geometries that have lone pair.
All the geometries with 1 lone pair is unsymmetrical
since the electron distribution is not even
in these geometries. For geometries with 2
lone pairs, angular and T-shape has unsymmetrical
lone pairs. As for the 3 lone pairs, the electron
distribution in T-shape is not symmetrical.
All these geometries highlighted in red, contains
unsymmetrical lone pair on the central atom.
That means, the lone pairs in square planar
and linear geometry, they are distributed
equally, opposite and away from each other,
and that makes it a symmetrical distribution
of the lone pairs. Now that we've clarified
what symmetrical and unsymmetrical lone pairs
look like, let's test out a few molecules
here. All of these molecules have the same
terminal atoms, so, we're gonna have to figure
out if they have symmetrical or unsymmetrical
lone pairs in order to decide if the molecule
is polar or not. Let's start with XeCl4. It
has square planar geometry, and the lone pair
is positioned opposite from each other, meaning
the lone pairs are symmetrical. NH3 has 1
lone pair, the shape is trigonal pyramid,
naturally, the lone pair is unsymmetrical
because it only has 1 lone pair. H2O has a
bent geometry where the 2 lone pairs are right
next to each other making the distribution
unsymmetrical. XeF2 has a linear geometry
where the 3 lone pairs are spreaded around
symmetrically. BrF3 has T-shape geometry with
2 lone pairs right next to each other. That
makes the electron distribution unsymmetrical.
If the molecule has unsymmetrical lone pairs
then it's called polar. If it has symmetrical
lone pairs, then it's called nonpolar. With
that, we've classified XeCl4 and XeF2 as nonpolar.
NH3, H2O and BrF3 as polar. Here are 2 more
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