Molecular Shapes
• Electron pairs are negative and repulse each other.
• valence-shell electron-pair repulsion theory (VSEPR)
• States that molecules will achieve a 3-D shape in order
to push electron-pairs as far away from each other as
possible
• Shape 1: Linear
• electron pairs force bonds to be 180º from each other
• Ex: Any molecule with only two atoms
• CO2
• Shape 2: Bent Linear
• Two pairs of unshared electrons push the bonds into a
bent shape
• Example: H2O
Molecular Shapes
Shape 3: (Trigonal) Pyramidal
– A single pair of unshared electrons push bonds away
into a pyramidal shape
Example: Ammonia, NH3
Shape 4: Tetrahedral
– With a central atom that bonds 4 times(Carbon), the
bonds are spaced out equally.
Example: Methane, CH4
Molecular Shapes
Shape 5: Trigonal Planar
• A central atom bonded to three elements with no
unshared pairs of electrons.
• Shape is triangular and flat
• Examples: BH3, CH2O
Molecular Polarity
· Polarity is the asymmetrical arrangement of charges in a
molecule
· Molecules can be polar or nonpolar based on their shape or
their symmetry.
Nonpolar molecules have symmetry.
– no positive/negative end
– charge is evenly distributed
Examples: Diatomic molecules(BrINClHOF), CH4, CO2
Polar molecules do not have symmetry
– Linear, trigonal planar and tetrahedral shapes can be
polar if there is no symmetry
– Bent linear and pyramidal shapes are always polar
Intermolecular Forces of Attraction
• Molecule- compounds that are formed by covalently bonded
elements
• Molecules tend to be
• soft or brittle
• Poor conductors of heat & electricity
• low melting points
• Many are gases or liquids at room temperature
• Intermolecular Forces of Attraction
• Determines the phase of molecules(solid, liquid or gas)
• Dipole-dipole forces- polar molecules tend to “stick” to each
other because of their positive-to-negative bonding.
• Van der Waals Forces- non-polar molecules have a weak
attraction to each other
• Dipole-dipole forces are stronger than Van der Waals forces
• Ionic bond > metallic bond > dipole-dipole > Van der Waals
Physical Properties of Molecules
“Like dissolves like” is a concept that explains why:
• Polar solvents dissolve polar solutes
• Nonpolar solvents dissolve nonpolar solutes
• The positive and negative ends of polar molecules line up
with each other, allowing polar solutes to dissolve.
• Nonpolar solutes will dissolve because their intermolecular
forces are weak, allowing them to dissolve.
• Nonpolar and polar do not mix because the intermolecular
forces of nonpolar solutes(aka Van Der Waals) cannot
overcome the intermolecular forces of polar solutes (dipoledipole forces).
Physical Properties of Molecules
· Ionic solutes can dissolve in polar solvents because polar
solvents will move their positive ends to the negative ion
and their negative end to the positive ion.
· This allows the ions to travel easily in a solution (and
conduct electricity)
 Ternary ionic compounds have both ionic and covalent
bonds.
 Polyatomic ions are made from covalent bonds, but can
gain or lose electrons to form an ion
Naming Binary Molecules
Step 1: Determine if ionic or molecular. If it starts with a metal, it is
ionic. See ionic rules for naming salts.
Step 2: The more metallic(closer to francium) atom is written first
Step 3: The least metallic (closer to flourine) atom is written
second & ending is changed to “-ide.”
Step 4: Add prefixes to indicate how many of each element is in
the formula. Mono is only used for the second element.
CO2 = carbon dioxide
# atoms Prefix
Examples:
– SO2
– CCl4
– N2H4
sulfur dioxide
carbon tetrachloride
dinitrogen tetrahydride
1
Mono
2
di
3
tri
4
tetra
5
penta
6
hexa
7
hepta
8
octa
9
nona
10
deca