Molecular Shapes & Polarity
The Valence shell electron pair repulsion theory
– VSEPR Theory
• The shape of a molecule or ion is governed by
the arrangement of the electron pairs around
the central atom.
• The pairs (sets) of electrons arrange
themselves to produce the minimum amount
of repulsion between them.
• You have to consider both bonding pairs and
lone pairs around the central atom.
Four electron pairs around the central
atom
• The simplest is methane, CH4. Determine its Lewis dot
diagram.
• Four electron pairs arrange themselves in space in what is
called a tetrahedral arrangement. A tetrahedron is a regular
triangularly-based pyramid. The carbon atom would be at
the centre and the hydrogens at the four corners. All the
bond angles are 109.5°.
• Build CH4 & draw it.
Other examples with four electron pairs around
the central atom
• Ammonia, NH3 – Determine it dot diagram
• Nitrogen is only forming 3 bonds, one of the pairs
must be a lone pair. The electron pairs arrange
themselves in a tetrahedral fashion as in methane.
• Build NH3 & draw it.
An Additional Factor
• Lone pairs are in orbitals that are shorter and rounder than
the orbitals that the bonding pairs occupy. Because of this,
there is more repulsion between a lone pair and a bonding
pair than there is between two bonding pairs.
• That forces the bonding pairs together slightly - reducing the
bond angle from 109.5° to 107°.
Remember this:
• Greatest repulsion
lone pair - lone pair
•
lone pair - bond pair
• Least repulsion
bond pair - bond pair
• When you describe the shape, you only take notice
of the atoms. Lone pairs are ignored although they
influence the shape.
• Ammonia is pyramidal - like a pyramid with the
three hydrogens at the base and the nitrogen at the
top.
Water, H2O
• Draw the dot diagram for H2O
• Following the same logic as before,
you will find that the oxygen has
four pairs of electrons, two of
which are lone pairs.
• These will again take up a
tetrahedral arrangement. This time
the bond angle closes slightly more
to 104°, because of the repulsion
of the two lone pairs.
• The shape isn't described as
tetrahedral, because we only "see"
the oxygen and the hydrogens not the lone pairs. Water is
described as bent or V-shaped.
Molecular Shapes Quiz
Formal
Central Central
Dot
Formula
Shared Unshared Shape
Diagram
Sets
Sets
(& why?)
NF3
H2S
NO2-1
Electronegativity
• Electronegativity is the ability of an atom to attract
electrons to form a chemical bond. An atom's
electronegativity is related to its electron affinity and
ionization energy.
• Electron affinity is the amount of energy that is
liberated by a gaseous element when an electron is
added to it.
• Ionization energy is the smallest amount of energy
required to remove the most weakly bond electron
from an atom of a gas.
What happens if two atoms of equal
electronegativity bond together?
• Consider a bond between two atoms, A and B. Each atom may
be forming other bonds as well as the one shown - but these
are irrelevant to the argument.
• If the atoms are equally electronegative, both have the same
tendency to attract the bonding pair of electrons, and so it
will be found on average half way between the two atoms. To
get a bond like this, A and B would usually have to be the
same atom. You will find this sort of bond in, for example, H2
or Cl2 molecules.
• This sort of bond could be thought of as being a "pure" covalent bond where the electrons are shared evenly between the two atoms.
What happens if B is slightly more
electronegative than A?
• B will attract the electron pair rather more than A
does.
• That means that the B end of the bond has more
than its fair share of electron density and so
becomes slightly negative. At the same time, the A
end (rather short of electrons) becomes slightly
positive. In the diagram, “d" (read as "delta") means
"slightly" - so d+ means "slightly positive".
Defining polar bonds
• This is described as a polar bond. A polar
bond is a covalent bond in which there is a
separation of charge between one end and
the other - in other words in which one end is
slightly positive and the other slightly
negative. Examples include most covalent
bonds. The hydrogen-chlorine bond in HCl or
the hydrogen-oxygen bonds in water are
typical.
Side 10 Chapter 20
Soaps & Detergents
•
•
•
•
•
Polar
Nonpolar
Hydrophilic
Hydrophobic
micelle
• (fear of water) doesn’t
dissolve in water
• molecule with no charges
• (water loving) dissolves in
water
• molecule with partial (+) &
(-) charges
• When soap molecules
surround a oil drop with
their hydrophobic tails
inside & hydrophilic heads
outside
What happens if B is a lot more
electronegative than A?
• In this case, the electron pair is dragged right
over to B's end of the bond. To all intents and
purposes, A has lost control of its electron,
and B has complete control over both
electrons. Ions have been formed.
A "spectrum" of bonds
• The implication of all this is that there is no clear-cut division
between covalent and ionic bonds. In a pure covalent bond,
the electrons are held on average exactly half way between
the atoms. In a polar bond, the electrons have been dragged
slightly towards one end.
• How far does this dragging have to go before the bond counts
as ionic? There is no real answer to that. You normally think of
sodium chloride as being a typically ionic solid, but even here
the sodium hasn't completely lost control of its electron.
Because of the properties of sodium chloride, however, we
tend to count it as if it were purely ionic.
Summary
• No electronegativity difference between two
atoms leads to a pure non-polar covalent
bond.
• A small electronegativity difference leads to a
polar covalent bond.
• A large electronegativity difference leads to an
ionic bond.
Polar bonds and polar molecules
• In a simple molecule like HCl, if the bond is polar, so also is the whole
molecule. What about more complicated molecules?
• In CCl4, each bond is polar.
• The molecule as a whole, however, isn't polar - in the sense that it doesn't
have an end (or a side) which is slightly negative and one which is slightly
positive. The whole of the outside of the molecule is somewhat negative,
but there is no overall separation of charge from top to bottom, or from
left to right.
By contrast, CHCl3 is polar.
• The hydrogen at the top of the molecule is less
electronegative than carbon and so is slightly
positive. This means that the molecule now has a
slightly positive "top" and a slightly negative
"bottom", and so is overall a polar molecule.
• A polar molecule will need to be "lop-sided" in some
way.
Patterns of electronegativity in the
Periodic Table
• The most electronegative element is fluorine.
If you remember that fact, everything
becomes easy, because electronegativity must
always increase towards fluorine in the
Periodic Table.
Polarity & Solubility Quiz
Formula
NH3
CS2
(EN,,d)
Dot
Formal
Diagram Shape
(EN,,d) Name
(why?)
Polar or
Soluble in
Nonpolar? water or
oil?
Side 6 Chapter 20 Solutions – Ion-Dipole
Interactions & Solvation
• Ion = charged particle (atom or combination of
atoms)
• Dipole = 2 poles of molecule, (-) & (+)
• Ion – dipole interaction = Example: water molecules
pulling apart the NaCl crystal and surrounding each
ion “solvation”
• Solute = substance being dissolved (lesser ingredient)
Ex: salt
• Solvent = usually a liquid that dissolves the solute
(major ingredient) Ex: water
Side 6 Chapter 30
Polar & Nonpolar Solvents
•
•
•
•
•
•
•
•
Immiscible = will not mix or dissolve
Miscible = will mix or dissolve
H20 polar
Carbon tetrachloride?
CCl4 symmetrical & nonpolar
“Like dissolves like”
I2 dissolves in CCl4
CuCl2 dissolves in H2O