Unit 7 Molecular Geometry

Essential Question What is a Lewis structure and what does it tell us?

Lewis Structures Properties are determined by molecular shape Lewis structures can be used to determine shape Shows the arrangement of electrons in a compound

Writing Lewis Structures Step 1: Add up valence electrons Ex: H2O H = 1 valence electron X 2 atoms O = 6 valence electrons 8 valence electrons

Writing Lewis Structures Step 2: Determine the central atom If Carbon is in the compound, it is the central atom If Carbon is NOT present, the LEAST electronegative element is central Hydrogen is NEVER the central atom

Writing Lewis Structures Step 3: Draw a pair of electrons between the central atom and each bonded atom A pair of bonding electrons can be represented by a dash or two dots between the bonding atoms H : O : H or H – O – H

Writing Lewis Structures Step 4: Fill in the rest of the electrons to satisfy the octet rule for all elements in the compound Hydrogen must satisfy the duet rule .. .. H : O : H or H – O – H .. ..

Writing Lewis Structures Step 5: Count up all the electrons. Make sure everything has an octet (duet for hydrogen). If you always check your work, you can’t make a mistake!! HINT: For these instructions and more helpful tips see page 22 in your packet 

Writing Lewis Structures If necessary, use two pairs of electrons to form a double bond between two atoms Ex: CO2

Writing Lewis Structures If necessary, use three pairs of electrons to form a triple bond between two atoms Ex: N2

Essential Question What is polarity?

Polarity Polar bonds are an intermediate between covalent bonds (equal sharing) and ionic bonds (complete transfer of electrons)

Polarity and Electronegativity Recall from previous units… Electronegativity is the ability of an element to attract elements in a compound High = very attracted to electrons Low = not so attracted to electrons Some values can be located on page pg 21 in your packet.

Polarity and Electronegativity You can determine if a bond is nonpolar covalent, polar, covalent or ionic using electronegativity values (see pg 21 in your packet) How big is the difference? 3.3 1.7 0.3 0.0 Ionic Polar Covalent Nonpolar Covalent

Now we can determine Lewis structure for a molecule and polarity for a bond Weren’t we looking for shape??

What is molecular geometry? Essential Questions What is molecular geometry? What determines basic molecular geometry?

VSEPR Valence Shell Electron Pair Repulsion

VSEPR VSEPR theory determines the molecular geometry of the molecule Molecular Geometry – 3D arrangement of a molecule’s atoms in space Number and type of electron pairs determines shape

VSEPR We can classify as bonding pairs or lone pairs Bonding Pair – shared between two atoms Lone Pair – unshared electrons Number of each type of electron pair determines shape

VSEPR Molecular geometry determines the bond angle Electron pairs avoid each other, so they will stay as far apart as possible See page 21 in your packet

VSEPR (reference pg 17 or 21 in your packet)

VSEPR To use determine the shape: Draw Lewis Structure Either A - Count the number of lone and bonding pairs, and compare to the chart on the previous slide (this would require you to MEMORIZE the chart) or B - Compare to the chart on page 21 in your packet (that’s what you get on the test)

Molecular Polarity Some molecules that have polar bonds are nonpolar Molecular shapes can cause polarity to cancel out

Molecular Polarity Polar molecules are said to have dipoles The dipoles tell you which parts of the molecule are slightly negative and slightly positive

Molecular Polarity For BINARY Compounds Nonpolar Shapes Polar Shapes Tetrahedral Linear Trigonal Planar Polar Shapes Pyramidal Bent

Molecular Polarity Ternary Compounds will generally be polar Different elements have different electronegativities, so when there are more than two types of atoms the molecule will not be symetrical

What Does All This Mean?!? Polarity determines several properties

“Like Dissolves Like” Polar substances can be dissolved by other polar substances Nonpolar substances can be dissolved by other nonpolar substances Polar substances will NOT dissolve in nonpolar substances

“Like Dissolves Like” When two liquids are capable of mixing, they are miscible Immiscible means that they will not mix

“Like Dissolves Like” Water (H2O) is a POLAR substance So is glass (SiO2) When water is in a glass container, why does the meniscus curve up?

“Like Dissolves Like” We know glass is POLAR Hexane is NONPOLAR Explain the behavior of this meniscus

“Like Dissolves Like” Is density the only reason why your salad dressing separates??

More Properties

More Properties

What type of bear dissolves in water??