Lewis Dot Structures on Board
Ionic
uses charges (NaCl)
Covalent
1: Find total # Valence e2: Each H gets 2, all others get 8
3: Line indicates shared pair of eH2O – single bond
CO2 – double bond
N2 – triple bond
Single Bonds
___________
Double Bond
___________
Triple bond
Lewis Dot diagrams
Practice
Molecular Geometry
Seeing Molecules in 3-D
Molecular Geometry
molecules are really 3-D!
CH4 in 2-D on a
sheet of paper
CH4 looks like
this in 3-D
HOW DO WE KNOW THE SHAPE OF MOLECULES?
X-RAY CRYSTALLOGRAPHY
Valence Electrons determine Molecular
“VSEPR” Shape
• VSEPR = “Valence-Shell Electron Pair
Repulsion”
• Electron pairs (bonding or lone pairs) in a
molecule repel each other and will try and
get as far away from each other as
possible… this determines the shape.
Lone pair electrons
bonding pair electrons
NH3 in 2-D
NH3 VSEPR shape
in 3-D
4 Shapes to Know
Tetrahedral
Trigonal
Pyramidal
Bent
Linear
How Lone Pairs Affect
Molecular Shape
“paddles” are lone pairs of electrons.
Remove the paddles and you can see the shapes.
Molecular Geometry
Shapes in Large Molecules
Large molecules
are composed
of the small
shapes we’ve
studied
Ex: tetrahedral
Why Shape Matters. SMELL
Ethyl Acetate
(C4H8O2)
Butyric Acid (C4H8O2)
Same formula, but different shapes
= very different smells
Rum extract smell
Rancid butter smell
Why Shape matters. TASTE (Sugar)
Why Shape matters (drugs)
Your body acclimates to drugs and it takes
more and more to get the same feeling.
Molecular Polarity & Smell
• Besides shape, polarity
also plays a role in your
ability to smell.
– Polar molecules = smell
– Non-polar = don’t smell
• Your smell receptors are
polar and surrounded by
mucous (a watery
substance)
Ex: Methane gas is
odorless
-They add this polar
stinky chemical to it
so that you can smell
it:
Determining Molecular Polarity
1. Draw correct VSEPR Shape
2. Determine if molecule is
symmetrical.
3. If the molecule is symmetrical =
non-polar
- no partial charges are needed!
4. If the molecule is NOT
symmetrical = polar
- you must show partial charges.
Examples of Polar &
Nonpolar Molecules
Molecular shape and Lewis Dot
diagrams
Practice
-Intramolecular forces = Bonding
within a molecule
“Atom to Atom”
-Intermolecular forces
“Molecule to Molecule”
2 main types
-Intermolecular forces
1) London Dispersion Forces
2) Dipole – Dipole Forces
Hydrogen Bonding
Without these forces there
would be no liquids or solids.
Everything would be a gas.
1) London Dispersion forces
Temporary very weak attraction between
electron clusters in Noble gases and
nonpolar molecules.
There is nothing else holding them
together.
London Dispersion forces
2) Dipole-Dipole forces
Attraction between the
positive and negative
parts of different
molecules.
Like Dissolves Like
NaCl (Polar) dissolved in Water (Polar)
Like Dissolves Like
Oil is non-polar
How does soap work?
Polar and non-polar ends
50 ml water plus 50 ml alcohol = ? ml
Water
Hydrogen Bonding due to polarity
Positive Hydrogen end attracted to Negative Oxygen end
Water
Hydrogen Bonding due to polarity
Positive Hydrogen end attracted to Negative Oxygen end
Water - Hydrogen Bonding
Ice density - Water is most dense at 4oC
Water
Hydrogen Bonding due to polarity
Cohesion – Surface Tension
Properties of Water due to Hydrogen
Bonding & Polarity
•
Cohesion – water molecules
are attracted to one another
– Causes water to be “Sticky”
– This is why water forms
droplets
•
Adhesion – water is
attracted to other substances
– Water will “stick” to
containers & objects
•
•
Surface tension – strong
forces between molecules
cause the surface of a liquid
to contract
Capillary action- water is
attracted to other water
molecules and will “rise”
“USGS Water Science for Schools: All about water!” US Geological Survey. 9 December 2011. http://ga.water.usgs.gov/edu/index.html
More properties…
•
Capillary Action – the
movement of water within the
spaces of a porous material
due to the forces of adhesion,
cohesion, and surface tension.
•Universal Solvent things
dissolve in water- polarity
“USGS Water Science for Schools: All about water!” US Geological Survey. 9 December 2011. http://ga.water.usgs.gov/edu/index.html
Hydrogen Bonding due to polarity
Viscosity
Nitrogen
Hydrogen Bonding due to polarity
Positive Hydrogen end attracted to Negative Nitrogen
end
Hydrogen Bonding in Kevlar
Hydrogen bonding in
Kevlar, a strong
polymer used in bulletproof vests.
Hydrogen Bonding in DNA
Effect on melting & boiling point.
If the intermolecular forces are strong, you need more energy--and thus a
higher melting point--to overcome them.
Explanation:
Meaning of melting point: The temperature at which a substance changes
state from solid to liquid.
So what is the purpose of intermolecular forces of attraction ?
Holding two molecules together, which helps to keep the substance in a given
state. If the intermolecular forces are strong, you need more energy to
overcome them.
In the case of melting points, the particles in a solid are held by
intermolecular forces. To convert the solid into a liquid, you must overcome
the intermolecular forces.
Melting
• Some attractive forces holding the particles together are
broken and particles move freely around each other but are
still close together. The stronger these forces are, the more
energy is needed to overcome them and the higher the
melting temperature.
Boiling
• The remaining attractive forces are broken so the particles
can move freely and far apart. The stronger the attractive
forces are, the more energy is needed to overcome them and
the higher the boiling temperature.
Transfer Task