Teacher Lecture Notes
8-1 The Shape of Small Molecules
Molecular Geometry – shows shape of molecule
VSEPR(valance shell electron pair repulsion) Theoryvalance electrons are arranged as far apart from each other as
possible. Why?
Molecular Shapes
Linear- 1D. Atoms in a straight line. Bond
angles 180o
Carbon dioxide
Student Lecture Notes
State Standards
C4.3e Predict whether the
forces of attraction in a solid
are primarily metallic,
covalent, network covalent,
or ionic based upon the
elements’ location on the
periodic table.
C4.4b Identify if a molecule is
polar or non-polar given a
structural formula for the
compound.
Trigonal Planar- 1D. Flat triangular. Bond angles
o
120
Boron Trichloride
Tetrahedral-3D. Bond angle 109.5o (four
surfaces!…Why?)
Methane
Pyramidal- Central atom with unshared pair and
three bonds (unshared pair exerts greater force!…Why?)
Bond angle 107o
ammonia
Bent- Central atom 2 unshared pairs..which do
what? 105o. Water
C4.3g Given the structural
formula of a compound,
indicate all the intermolecular
forces present (dispersion,
dipolar, hydrogen bonding).
Try These. Write formula if not given, then Lewis Dot, then
model
Ethyne (C2H2)
Formaldehyde (CH2O)
Nitrogen Trichloride
Ethane (C2H6)
Hybrid Orbitals-cross of two types of orbitals (s, p, d, f)
sp – (two orbitals; an s and p combine) linear 180o BeF2.
sp2- ( three orbitals; an 1s and 2p’s) trigonal planar. 120o.
sp3- (4 orbitals or pairs of electrons; no db. 1s and 3p orbitals)
tetrahedral 109.5o .
Try to predict the shapes based on the hybrid orbitals
Boron Trichloride
Methane
Water
Bond Length-two important trends
 Moving down a group bond length increases. Why?
 Multiple bonds are shorter than single bonds. Why?
Section Review 8-1
1. What is the VSEPR theory and how does it explain
molecular shapes of small molecules?
2. Draw and describe the shape of the five common
small molecules, include bond angles. Give an
example of each.
3. Describe two factors that influence bond length.
What periodic trend exists for bond length?
8-2 Polarity
Unequally shared electrons based on elements
Electronegativity. More electronegative atom has a slight
neg charge, the other atom therefore will be more
____________. Polar molecules are called dipoles. Dipoles
will align in a magnetic field and will attract to a charged
field. Why?
The shape of a molecule and the polarity of its bonds together
determine whether a molecule is polar or non polar in a small
molecule
Lets try a few together.
Formaldehyde (CH2O)
First draw the structural formula, figure out its shape, then
look at the electronegativities (hint- chpt 7LN or pg 241 fig 719 text)
Who’s more electronegative?
Draw the dipoles on the structural model.
Carbon dioxide (very tricky!)
Properties of this molecule make it a gas at room temp!
Water (again tricky, make sure you look at waters true
C3.2b
Describe the relative strength
of single, double, and triple
covalent bonds between
nitrogen
atoms.
shape!)
Properties make it a liquid at room temp bc of the attractions!
Large Molecules-polarity determines shape
Proteins- large chains with some subunits that are polar
causing a 3D “Knot”
States of Matter
Bond Type
Ionic
Polar Covalent
Non polar Covalent
Attraction
BP/MP
C5.8C Recognize that proteins,
starches, and other large
biological molecules are
polymers.
C5.4e Compare the melting
point of covalent compounds
based on the strength of IMFs
(intermolecular forces).
State Matter
Isomers – same molecular formula, different structural
formulas
C4.3i Explain why ionic solids
have higher melting points
than covalent solids (e.g., NaF
has a melting point of 995°C
while water has a melting
point of 0°
C5.4c Explain why both the
melting point and boiling
points for water are
significantly higher than other
small molecules of
comparable mass (e.g.,
ammonia and methane).
C4.3c Compare the relative
strengths of forces between
molecules based on the
melting point and boiling point
of the substances.
C4.3d
Compare the strength of the
forces of attraction between
molecules of different
elements
(e.g., at room temperature,
chlorine is a gas and iodine is
a solid).
C5.8A Draw structural
formulas for up to ten carbon
chains of simple
hydrocarbons.
C5.8B Draw isomers for simple
hydrocarbons.
Hydrogen Bonding
H bonding with O
C4.3f Identify the elements
necessary for hydrogen
bonding (N, O, F).
H bonding with N
H bonding with F
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