32A Lab “Molecular Shapes”• Introduction to Lewis Structures

– Graphical variation on the “Octet Rule”

• Molecular Shapes– Electronic versus Atomic– Mutual electrostatic repulsion, VSEPR idea

• Materials of modern interest– Hydrocarbons, graphene, nanotubes

• Lab experiment– Chime Tutorial, Cabrillo exercises

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Electron Dot Diagrams

• G.N. Lewis idea (UC Berkeley)– Elegantly simple idea, but very instructive– Show each bonding electron as a dot

• As elements brought together, dots merge• Most stable configuration is filled shell

– 2 dots for Hydrogen (2s2 or [He] configuration)– 8 dots for most others (s2+p6, Octet rule)

• Methane example C(4dot) + 4*H (1dot)

– Can have more electron pairs than bonds• “lone pairs” are non-bonding electrons• Lone pairs occupy a geometrical position

– Are part of molecular shape consideration

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Lewis Structure (electron dot diagram) for ammonia Each of the 3 hydrogen atoms will share its electron with nitrogen to

form a bonding pair of electrons (covalent bond) so that each hydrogen atom has a share in 2 valence electrons (electronic configuration of

helium) and the nitrogen has a share in 8 valence electrons (electron configuration of neon)

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3 of Nitrogen’s 5 valence electrons shared with 3 Hydrogen atoms in Ammonia. “Lone Pair” electrons attract Hydrogen ion

Result is formation of the Ammonium ion

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Each oxygen will share 2 of its valence electrons in order to form 2 bonding pairs of electrons (a double covalent bond) so that each oxygen

will have a share in 8 valence electrons (electronic configuration of neon).

Lewis Structure (electron dot diagram) for the oxygen molecule

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Examples across the chart

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Lewis Dot Diagrams for elements

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Electron Dot Diagrams

• Lines between atoms are 2-electrons– One line equivalent to 2 dots

• 2 lines (double bond) equivalent to 4 dots• 3 lines (triple bond) equivalent to 6 dots

– Can rotate around one line (no interference)• 2 lines (double bond) restricts rotation• 3 lines (triple bond), no rotation, always planar

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Each of 4 carbon valence electronsshares an orbit with 1 from Chlorine

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Electron Accounting• “AE” is shorthand for Available Electrons

– Sum of valence electrons from elements– Final arrangement must have same AE count

• Arranging atoms– Hydrogen always at the ends (never center)– Most positive non-metal in center– Carbon in center for Organic compounds

• Arranging electrons– Hydrogen shares 2 electrons (full 1s2 shell)– Other atoms share 8 electrons (ns2 + np6)

• “Lone Pairs” complete the shell, but are unbonded• Lone pairs take up positions as if bonded

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Available electrons include 7 for Chlorine and 6 for Oxygen. By picking up an electron elsewhere, the Cl and O both

have full octets with a negative charge of -1 overall

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What kind of bonds?

• Bond order, how many ? – singles, doubles, triples

• Most books suggest trial & Error

• Alternative is a simple calculation– Compare AE with total including sharing– Difference/2 is number of bonds needed– Courtesy of Nisha, a Chem-1A student

• She got it from her high school AP-Chem class

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3 of 5 Phosphorus electrons share orbits with 7th Bromine electron

AE=(3*7)+5=26Nisha=4*8=32

Difference/2 = 6/2 = 3 bonds

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AcetyleneUsually written as triple bond

AE=(2*4)+(2*1) = 10Nisha=(2*8)+(2*2) = 20

Δ/2 = 5 bonds, so a triple is required

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Resonance StructuresTwo (or more) equivalent arrangements

Actual molecule perhaps a hybrid of the two

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Dipole Moment, a quick reviewSeparation of charge, allowing molecule to be

“twisted” by external electrical field (turn=“moment”)

Fluorine most electronegative, Francium the least negative.Pauling’s scale maximum is 4, also indicated by column height

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Arrow ConventionHead of arrow points to negative end(s) of molecule

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Symmetry negates momentMolecule with symmetrical charge separation

is NOT effected by external fields

3 representations for water

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What about Shapes?Paintings versus Sculptures

• Lewis Diagrams are 2-D on paper– Do not provide shapes in nature

• Need 3-D models to handle geometry– Molecular shapes define behavior

• Right hand and left hand molecules in biology• “Hydrogen Bonding” a key attribute of water

– Relies on angles and lone pair electrons

• Macro-molecules, chains, gels, drugs

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2D versus 3D in ArtSoyer painting versus Rodin Sculpture …

which is more realistic?

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Escher illusions 3-D exploration in 2-D space

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Escher 2-D illustration, executed in 3-D with Legos

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2-D illusion and 3-D realityHow can we apply this to Chemistry?

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How to represent 3D molecules?

• Pictures and physical models– Drawings of “ball & stick” or “balloon” models– 3-D constructions using tinker-toys, legos, etc.

• Other forms of analogies and models– Computer simulations– Objects from other fields– Mathematical shapes

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Another 3-D representation in 2DDistortion inevitable, like Mercator Projection of globe

Mercator Projectionviewing a sphere as a flat object creates large distortions at

top and bottom, try flattening out an orange peel !

Mathematical Shapes

• Geometry provides a number of convenient shapes we can use

• Many molecules are examples from mathematical ideas

• There are 5 “Platonic Solids”, we will consider many of these as molecules

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Tetrahedron• A regular tetrahedron is composed of

four triangular faces, three of which meet at each vertex. This example with = four "equilateral triangles, " is one of the 5 Platonic solids.

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Shape of Ammonium is Tetrahedral4 hydrogen ions repel each other, most equidistant

arrangement is Tetrahedral, 109 degree anglesSame geometry applies for Methane, CH4

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Electronic vs Mechanical shapes

• Electronic configuration define spatial positions– Electrons avoid each other, max. separation– Common electronic shapes

• Linear, planar, tetrahedral, pyramidal, octahedral

• Mechanical (molecular), is where atoms are– Not all electronic positions have atoms– Lone pairs define shape, but no atom there– Common molecular shapes

• Linear, planar, tetrahedral, pyramidal, octahedral• missing atoms bent (water), see-saw, square pyramid

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VSEPR is a useful model(Valence Shell Electron-Pair Repulsion)

• Like-charge atoms push each other away– Repelling atoms try for maximum separation

• Max. separation varies by number of items• 2 items repelling => 2-D linear shape, 180o

• 3 items repelling => 2-D trigonal shape, 120o

• 4 items repelling => 3-D tetrahedral shape, 109o

• 5 items repelling => trigonal bi-pyramid, 120o & 90o

• 6 items repelling => 3-D octahedral shape, 90o (same as “square bi-pyramid”)

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Electron AvoidanceLike Charges repel (electrons are all negative) …

so electrons put maximum distance between each other

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VSEPR Model

• “Valence Shell Electron Pair Repulsion” = VSEPR– Basic idea is that electrons within orbitals repel each other– Orbital repulsion gives rise to specific directions for bonding– Specific bonding directions lead to 3-D shapes of molecules

• 2-Cloud case– Repulsion forces in-line orientation, 180 degree separation– Minimum influence from most distant arrangement– Examples include O=C=O, H-C≡N

• 3-Cloud case– Maximum separation achieved in planar arrangement– Approximately 120 degrees between atoms, depends on charge– Example includes formaldehyde H2-C=O

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Balloon Illustration2 objects must have linear (1-D) arrangement

3 objects must have triangular (2-D) arrangement 4 object often have Tetrahedral (3-D) arrangement

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3-cloud case

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Triangular (planar) Symmetry3 single bonds equivalent (in shape)

to a mix of 2 single + 1 double

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VSEPR Models

• 4-Cloud case– Repulsion forces maximum separation into tetrahedron– Minimum influence between most distant electrons– Angle between any 2 is 109.5 degrees– Many examples, such as Methane CH4

– Lone pairs help define shape, but do not involve other atoms

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4-cloud case

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Methane, a 4-cloud case

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Tetrahedral Symmetry4 chlorines at apex of tetrahedron

Similar to Methane (CH4), and many otherscannot show accurately in 2-D, need a 3-D version

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VSEPR Model

• 5-Cloud case– Maximum separation achieved in 5 directions– 3 direction outward in a plane, plus above and below

• Called a “Trigonal Bi-pyramid” shape

– Most common with elements having 5 valence electrons• PCl5 is a typical example

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An example of trigonal bi-pyramid molecular geometry that results from five electron pair geometry is PCl5. The

phosphorus has 5 valence electrons and thus needs 3 more electrons to complete its octet. However this is an example where five chlorine atoms are present and the octet is expanded to 10. The Lewis diagram is as follows:

Cl = 7 e- x 5 = 35 e-P = 5 e- = 5 e-Total = 40 e-

The Chlorine atoms are as far apart as possible at nearly 90o and 120obond angle. This is trigonal bi-pyramid

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In this example, SF4, the Lewis diagram shows S at the center with one lone electron pair and four fluoride atoms attached, total of 10. Lewis diagram is as follows:

S = 6 e-F = 7 e- x 4 = 28 e-

Total electrons = 34 e-With four atoms and one lone pair, the electron pair geometry istrigonal bi-pyramid. The molecular geometry is called see-saw.

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VSEPR Model

• 6-Cloud case– Maximum separation in 6 directions is an

Octahedron• 4 clouds in a plane (as a square), plus above and below• 90 degree angle between any two faces or cloud directions

– Another very common configuration• Uranium hexaflouride for isotope separation by gas diffusion

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6-Cloud Case• Octahedral shape

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Octahedron• A regular octahedron is a Platonic solid composed of eight

equilateral triangles as faces, four of which meet at each vertex.

• The regular octahedron is a special kind of triangular antiprism and of square bipyramid. The regular octahedron has 6 vertices and 12 edges

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Uranium Hexafluoridea gas used for separating isotopes via diffusion

example of octahedral shape, 6 clouds

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6 Cloud case (less one)example is Bromine pentafluoride

• Square Pyramidal has 6 lobes, but one is non-bonding and unseen in BrF5

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Can have word combinations …such as “trigonal planar”

Trigonal around each carbonDouble bond denies rotation, forces planar shape

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Summary so far …• Lewis Diagrams, a 2-D structure model

– One dot = 1 electron– One dash = 2 electrons (single bond)– Two dashes = 4 electrons (double bond)– Three dashes = 6 electrons (triple bond)– Lone pairs = 2 electron surplus, NOT bonded

• Attracts (+) species, often H+ “Hydrogen Bond”

– More positive atom in molecule center• But hydrogen always on the outside perimeter

– All atoms (not hydrogen) surrounded by 8 electrons• Electrons can be transferred, shared, or in lone pairs = 8

total• Hydrogen always surrounded by 2 electrons (full 1s2 shell)

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Summary so far …• VSEPR (Valence Shell Electron-Pair Repulsion)

– Electron (-) charge repels all other electrons– Repulsion leads to maximum separation of (-) species– Maximum repulsion leads to geometric shapes.

• Geometric models from Mathematics– Basic 2-D & 3-D shapes found in Geometry

• 2 repulsion items = a line, 180o separation (2-D)• 3 repulsion items = a triangle, 120o separation (2-D)• 4 repulsion items = tetrahedron, 109.5o (3-D)• 5 repulsion items = trigonal bi-pyramid, 90o & 120o (3-D)• 6 repulsion items = square bi-pyramid (Octahedron), 90o

– Works exactly per the model with single bonds• Lone pair angles deviate slightly

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Multiple Bonds• Single, Double, and Triple bond shapes

– All bond species form equivalent bond angles– Consider 8 electrons (4 electron pair) for Carbon

• 4 single bonds = tetrahedral– Methane, Carbon Tetrachloride, the most common shape

• 2 single + 1 double = 3 bond separation = Triangular– Formaldehyde H2C=O

• 2 double = 2 bond separation = linear– Carbon Dioxide O=C=O

• 1 Triple + 1 single = 2 bond separation = linear– Acetylene H-C≡C-H

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Dot Structures and Resonance

• “Equivalent but Different” results can occur– Ozone O=O-O, and O-O=O are equivalent– Sulfur dioxide O-S=O, and O=S-O are equivalent

• Not stereo-isomers, no chemical difference, same stuff• Both considered correct simultaneously

– Resonance• Our system of dots and lines is an accounting tool

– Nature does not worry about “counting the dots”

• Electrons resonate back and forth between the bonds– Amounts to electron sharing between equivalent configurations– An accountant might call it 1.5 bonds (hard to conceptualize)

• The overall system is considered a “Resonance Hybrid”– Bond lengths the same from either end– Bonds are entirely equivalent– Another example of electron sharing

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Summary on Shapes

• Electronic Structures & Shapes– Dominated by electron behavior– Explains how repulsion dictates geometry– “Lone Pairs” contribute to shapes, geometry

• Molecular Structures– Shapes from “Real Atoms”, not electrons– What a atom-sized person would see– Shapes with missing parts

• You see the atoms in a molecule • You can’t observe electrons

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Summary on Shapes• Electronic Structure of water is Tetrahedral

– Oxygen has 4 electron “arms” as SP3 bonds– Two arms attached to hydrogen + 2 lone pairs

• Molecular Structure of water is Bent– 2 hydrogens engage two arms of tetrahedron– 2 lone pairs occupy other 2 arms … felt but unseen

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Organic Molecules

• Organic ≡ Carbon based– Most numerous class of materials– Countless combinations are possible– Carbon bonds with almost any other element

• Carbon to carbon bonds very common– Gases (acetylene, propane)– Linear chain liquids (gasoline, oil)– Ring structures (benzene, cyclopentane)– Crystalline structures (diamond, graphite)

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Basic shapes for Organics• Carbon is electronically tetrahedral

– Implication is NOT a straight line formation

• Real hydrocarbons zig-zag at 109o

• Ring structures are common– Benzene, cyclohexane

• Recently discovered new shapes– Buckeyballs (found in soot)– Graphene sheets (in pencil lead)– Carbon Nanotubes, fibers

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We draw straight lines for convenience, it’s easierReal molecules hve tetrahedral carbon, zig-zag shaped

Fig. 13-12, p. 382

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Hydrocarbons

• Alkane series, linear progression of carbon– Methane = 1 carbon (natural gas)– Ethane = 2 carbon (ripening fruit)– Propane = 3 carbon chain (gas BBQ)– Butane = 4 carbon chain (cigarette lighters)– Pentane = 5 carbon chain– Hexane = 6 carbon chain (Naptha)– Heptane = 7 carbon chain– Octane = 8 carbon chain (gasoline reference)

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Octane and Iso-OctaneZig-Zag shape from tetrahedral bonding

“Buckeyballs” named for Buckminster Fuller60 carbons in sockerball shape, “C60” a closed surface hexagon & pentagon mix

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Graphene, a 2-D array of carbon excellent electrical conductor, very strong

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DIY graphene on a DVD playerhttp://www.chemistryviews.org/details/news/1662993/DVD_Player_Produced_Graphene_Films.html

Carbon Nanotubes, graphene rolled up like “chicken wire”, some are semiconductors

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Computer Modeling

• CABRILLO COLLEGE has an interactive molecule display, allowing users to twist and turn common molecules to investigate their structures. Almost like a video game, but more educational than killing aliens. We will use this in lab class, lots of options to investigate, plus some answers

• http://c4.cabrillo.edu/chem30a/exercises/Exer_1/index.html

• http://c4.cabrillo.edu/chem30b/exercises/chpt_11_060700/index.html

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Cabrillo website interactive exercises, part 1 is on Lewis Structures, draw them out

Answers to selected items also provided

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Section 2, shapes of ions, You draw Lewis dot structures & name the shape

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Section 3, Polar & Non-Polar materialsYou will identify which is which

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Section 4: show structure, charge distribution, and if polar

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Section 5, naming compounds

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Section 6, naming ions

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Section 7, involves nomenclaturecombine anion in picture with cations in the list

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Section 8 (last one)Match the ion with the name and give its charge

Computer Lab reserved for us• Do the Chime Tutorial first

– Use Internet Explorer to load it• Google Chrome, other browsers do NOT work

– Answer questions in lab supplement– Can finish this at home … but with extra effort

• Requires JAVA program, free download• Requires Chime program, copy available here

• Do Cabrillo College exercises after tutorial– Start each section, ask questions– Can finish at home, need only JAVA– See Jaguar posting for details

More instructions are on Jaguar

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