Use VSEPR theory to predict molecular geometry and connect bond polarity, molecular shape, and intermolecular forces to physical properties.
A lesson of Chemistry, in Chemical Bonding and Molecular Structure, about 40 minutes.
By the end of this lesson, you will be able to predict a molecule's shape from its electron groups, tell a polar molecule from a nonpolar one even when its bonds are polar, and use that to predict how a substance behaves.
Electron Groups Repel and Spread Out
That is the whole of VSEPR theory: valence shell electron pair repulsion.
Everything around a central atom pushes everything else as far away as possible in
three dimensions.
Count electron GROUPS, not atoms. A group is any of:
a single bond
a double bond (counts as ONE group)
a triple bond (also one group)
a lone pair
A double bond counts as one because both pairs sit between the same two
nuclei, so they push as a single region of negative charge.
Groups
Arrangement
Angle
$2$
linear
$180°$
$3$
trigonal planar
$120°$
$4$
tetrahedral
$109.5°$
Those arrangements are the geometry of the ELECTRONS. The shape you name is the
arrangement of the ATOMS, and when there are lone pairs the two are different.
Lone Pairs Are Invisible and Pushy
A lone pair occupies space and repels, but it is not an atom, so it does not
appear in the molecular shape's name.
All three have four electron groups arranged tetrahedrally. They look completely
different because you only see the atoms.
And lone pairs push harder than bonds. A bonding pair is pulled between two
nuclei and stays fairly confined; a lone pair is held by one nucleus only and
spreads out. So each lone pair squeezes the remaining bond angles down by a couple
of degrees, which is exactly why the angles fall from $109.5$ to $107$ to $104.5$.
Three groups behave the same way: no lone pairs gives trigonal planar, one
lone pair gives bent at about $118°$.
Sort It: What Shape
Build four bonded atoms and no lone pairs, then remove one bond and add a lone pair in its place. Watch what happens to the angle between the remaining bonds.
The page is flat and molecules are not, and no diagram substitutes for rotating
one. Set a central atom, add bonded atoms, then convert a bond into a lone pair
and watch the angle close up.
The three shapes you just built are methane, ammonia and water: the same
tetrahedral electron arrangement, seen three different ways because the invisible
groups changed.
Example 1: Predicting a Shape
Here is every step, worked all the way through.
$$\mathrm{H_2O}$$
How many valence electrons does oxygen have? 6
Two are used in bonds to hydrogen. How many electrons are left over? 4
Four leftover electrons make how many lone pairs? 2
Two bonds plus two lone pairs is how many electron groups? 4
Four groups with two lone pairs gives which shape? Type one word bent
Bond Polarity and Molecular Polarity
Two different questions, and confusing them is the error this lesson exists to
prevent.
A bond is polar when the two atoms have different electronegativities, so the
shared electrons sit closer to one of them. The bond has a partial negative end
and a partial positive end.
A MOLECULE is polar when those bond dipoles do not cancel out.
Both bonds are strongly polar. The molecule is nonpolar, because it is linear
and the two dipoles point in exactly opposite directions and cancel.
$$\mathrm{H_2O}$$
The same two polar bonds, but the molecule is bent, so the dipoles do not oppose
each other. They add to give a net dipole, and water is strongly polar.
Dipoles are vectors, and geometry decides whether they cancel. That is why
this lesson had to come after shapes: polarity is not readable from the formula.
The symmetry rule: if the shape is symmetric AND all the outer atoms are the
same, the dipoles cancel and the molecule is nonpolar. Break either condition and
it is polar.
Both are tetrahedral. Replacing one hydrogen with chlorine destroys the symmetry
of the pull, and the cancellation fails.
Sort It: Polar or Nonpolar Molecule
Worked Example 1: Bonds Polar, Molecule Not
$$\mathrm{CCl_4}$$
Worked Example 2: A Lone Pair Breaking Symmetry
$$\mathrm{NH_3}$$
Why Polarity Decides Behavior
$$\textbf{like dissolves like}$$
Polar solvents dissolve polar and ionic solutes; nonpolar solvents dissolve
nonpolar ones. Water dissolves salt and sugar and refuses oil, and oil dissolves
grease.
Polar molecules attract each other more strongly, so they need more energy to
separate:
Substance
Molar mass
Boiling point
$\mathrm{CH_4}$, nonpolar
$16$
$-161°C$
$\mathrm{H_2O}$, polar
$18$
$100°C$
Almost identical masses, and a difference of $261$ degrees. That gap is entirely
polarity, and specifically hydrogen bonding, the exceptionally strong
attraction that appears when hydrogen is bonded to N, O or F.
Water's bent shape is why life exists. If water were linear it would be
nonpolar, it would boil around $-80°C$, ice would sink, and it would dissolve
almost nothing. Every property that makes water a biological solvent comes from
two lone pairs bending it by $75$ degrees.
Worked Example 3: Predicting Solubility
Common Mistakes
Mistake 1: Counting a double bond as two groups
It is one region of electron density, so one group.
Mistake 2: Naming the electron arrangement as the shape
Water's electrons are tetrahedral; its shape is bent.
Mistake 3: Assuming polar bonds mean a polar molecule
$\mathrm{CO_2}$ has two polar bonds and no net dipole.
Mistake 4: Forgetting to check the outer atoms are identical
$\mathrm{CH_3Cl}$ is tetrahedral and polar.
Mistake 5: Ignoring lone pairs when counting groups
They are invisible in the shape and fully present in the count.
Where This Goes Next
Shape is not a detail of a molecule. In biology it is very nearly the whole
story.
Enzymes work by shape. An enzyme's active site fits one substrate the way a
lock fits one key, and a molecule with the right formula and the wrong geometry
does nothing at all. Most drugs work by fitting a protein's binding site, which is
why drug design is largely three-dimensional modeling rather than chemistry in
the formula-writing sense.
Handedness makes this brutally concrete. Two molecules can be mirror images
with identical formulas and identical bonds, and behave completely differently
because a receptor is itself handed. Thalidomide's two forms differed in exactly
this way: one treated morning sickness and the other caused severe birth defects,
and the tragedy in the late 1950s is why every chiral drug is now tested as
separate forms.
Protein folding is the same idea at scale. A chain of amino acids folds into a
specific three-dimensional shape, that shape determines what the protein does, and
misfolding causes diseases including Alzheimer's and CJD. Predicting the fold from
the sequence was an open problem for fifty years until machine learning made real
progress on it.
Shape comes from electron repulsion, which comes from where the electrons sit,
which comes from the periodic table. The chain from "group 16 has six valence
electrons" to "water is bent, so life is possible" is short, and every link in it
is in this unit.
Check Your Understanding
Five of them, no instructions this time.
Summary
Count electron GROUPS, with a double or triple bond counting as one, and lone
pairs counting fully.
Lone pairs are invisible in the shape's name and push harder than bonds, which
is why the angles close from $109.5$ to $107$ to $104.5$.
Polar bonds do not make a polar molecule. Geometry decides whether the dipoles
cancel.
Symmetric shape plus identical outer atoms means nonpolar. Break either
condition and it is polar.
Like dissolves like, and water's bent shape is why it dissolves what it does.