Notes on Shape & review of
electronegativity and bonding.
Your Assignment-due 1/11/12
• 1. Finish the worksheet from Friday
• 2. Read Page 332-339 in your book-read
it carefully and go through all the practice
problems so you understand them.
• 3. Answer #1-3 from page 339
• 4. Read pg. 341
• 5. From pg. 342-343 do questions: 5, 9, 10,
13-20, 22
I was watching Ironman II
over the weekend, the
compound that helped
cure his blood toxicity was
lithium dioxide. Name 2
things wrong with the
name of this compound.
DMA #1
1/12/12
Calculate the percent
composition of: CaCl2
CH4
C3H8
DMA #2
1/13/12
What are the
7diatomic
elements? Hintlook on your
periodic table!
DMA #3
1/24/12
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 of electrons arrange themselves to
produce the minimum amount of repulsion
between them.
• In other words, they spread out as far as they
can!
• You have to consider both bonding pairs and
lone pairs.
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 its 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.
• Draw NH3
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
• Oxygen has four pairs of
electrons, two of which
are lone pairs.
• These make a tetrahedral
arrangement. T
• The bond angle closes
slightly 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
Three electron pairs around the central
atom
• The simple cases of this would be BF3 or BCl3.
• Boron is in group 3, so starts off with 3 electrons.
• It is forming 3 bonds, adding another 3 electrons.
There is no charge, so the total is 6 electrons - in
3 pairs.
• Because it is forming 3 bonds there can be no
lone pairs. The 3 pairs arrange themselves as far
apart as possible. They all lie in one plane at 120°
to each other. The arrangement is called trigonal
planar.
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 attract the
bonding pair of electrons, and so it will be found on average
half way between the two atoms.
• 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.
What happens if B is slightly more
electronegative than A?
• B will attract the electron pair 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.
• The A end (rather short of electrons) becomes
slightly positive.
• In the diagram, “d" (read as "delta") means
"slightly" - so d+ means "slightly positive".
Polar Covalent Bonding Animation
• http://www.tutorvista.com/content/chemistry
/chemistry-iii/chemical-bonding/polarcovalent-animation.php
Defining polar bonds
• This is described as a polar bond.
• A polar bond is a covalent bond 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.
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.
• So-it is called an ionic bond
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.