Unit 7Unit 7Molecular GeometryMolecular Geometry

Essential QuestionEssential QuestionWhat is a Lewis structure and what does it tell us?

Lewis StructuresLewis StructuresProperties are determined by

molecular shapeLewis structures can be used to

determine shape◦Shows the arrangement of electrons

in a compound

Writing Lewis StructuresWriting Lewis StructuresStep 1: Add up valence electrons

Ex: H2O

H = 1 valence electron X 2 atomsO = 6 valence electrons

8 valence electrons

Writing Lewis StructuresWriting Lewis StructuresStep 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 StructuresWriting Lewis StructuresStep 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 StructuresWriting Lewis StructuresStep 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 StructuresWriting Lewis StructuresStep 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 StructuresWriting Lewis StructuresIf necessary, use two pairs of

electrons to form a double bond between two atoms

Ex: CO2

Writing Lewis StructuresWriting Lewis StructuresIf necessary, use three pairs of

electrons to form a triple bond between two atoms

Ex: N2

Essential QuestionEssential Question

What is polarity?

PolarityPolarityPolar bonds are an intermediate

between covalent bonds (equal sharing) and ionic bonds (complete transfer of electrons)

Polarity and Polarity and ElectronegativityElectronegativityRecall 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 Polarity and ElectronegativityElectronegativityYou 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 PolarCovale

nt

Nonpolar

Covalent

Now we can determine Lewis structure for a molecule and polarity for a bond

Weren’t we looking for shape??

Essential QuestionsEssential Questions

What is molecular geometry?

What determines basic molecular geometry?

VSEPRVSEPRValenceShellElectronPairRepulsion

VSEPRVSEPRVSEPR 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

VSEPRVSEPRWe 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

VSEPRVSEPRMolecular geometry determines

the bond angleElectron pairs avoid each other,

so they will stay as far apart as possible

See page 21 in your packet

VSEPR VSEPR (reference pg 17 or (reference pg 17 or 2121 in your in your packet)packet)

VSEPRVSEPRTo use determine the shape:

1. Draw Lewis Structure2. 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)

orB - Compare to the chart on page 21 in your packet (that’s what you get on the test)

Molecular PolarityMolecular PolaritySome

molecules that have polar bonds are nonpolar

Molecular shapes can cause polarity to cancel out

Molecular PolarityMolecular PolarityPolar molecules

are said to have dipoles◦The dipoles tell

you which parts of the molecule are slightly negative and slightly positive

Molecular PolarityMolecular PolarityFor BINARY CompoundsNonpolar Shapes

◦Tetrahedral◦Linear◦Trigonal Planar

Polar Shapes◦Pyramidal◦Bent

Molecular PolarityMolecular PolarityTernary Compounds will

generally be polarDifferent elements have different

electronegativities, so when there are more than two types of atoms the molecule will not be symetrical

What Does All This What Does All This Mean?!?Mean?!?

Polarity determines several properties

““Like Dissolves Like”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”Like Dissolves Like”

When two liquids are capable of mixing, they are miscible

Immiscible means that they will not mix

““Like Dissolves Like”Like Dissolves Like”Water (H2O) is a POLAR

substanceSo is glass (SiO2)

When water is in a glass container, why does the meniscus curve up?

““Like Dissolves Like”Like Dissolves Like”We know glass is POLARHexane is NONPOLAR

Explain the behavior of this meniscus

““Like Dissolves Like”Like Dissolves Like”Is density the only reason why

your salad dressing separates??

More PropertiesMore Properties

More PropertiesMore Properties

What type of bear dissolves What type of bear dissolves in water??in water??