Lewis Dot Structures on Board Ionic uses charges (NaCl) Covalent 1: Find total # Valence e- 2: Each H gets 2, all others get 8 3: Line indicates shared pair of e- H2O – 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 VSEPR shape in 3-D NH3 in 2-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 Draw correct VSEPR Shape Determine if molecule is symmetrical. If the molecule is symmetrical = non-polar - no partial charges are needed! 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 -Intramolecular forces = Bonding within a molecule “Atom to Atom” -Intermolecular forces “Molecule to Molecule” 2 main types

-Intermolecular forces. 1) London Dispersion Forces -Intermolecular forces 1) London Dispersion Forces 2) Dipole – Dipole Forces Hydrogen Bonding

Without these forces there would be no liquids or solids 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 bullet-proof vests.

Hydrogen Bonding in DNA

Effect on melting & boiling point 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