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 electrons
shares 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=26
Nisha=4*8=32
Difference/2 = 6/2 = 3 bonds
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Acetylene
Usually written as triple bond
AE=(2*4)+(2*1) = 10
Nisha=(2*8)+(2*2) = 20
Δ/2 = 5 bonds, so a triple is required
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Resonance Structures
Two (or more) equivalent arrangements
Actual molecule perhaps a hybrid of the two
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Dipole Moment, a quick review
Separation of charge, allowing molecule to be
“twisted” by external electrical field (turn=“moment”)
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Fluorine most electronegative, Francium the least negative.
Pauling’s scale maximum is 4, also indicated by column height
Arrow Convention
Head of arrow points to negative end(s) of molecule
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Symmetry negates moment
Molecule with symmetrical charge separation
is NOT effected by external fields
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3 representations for water
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 Art
Soyer 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 reality
How 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 2D
Distortion inevitable, like Mercator Projection of globe
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Mercator Projection
viewing 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
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 Tetrahedral
4 hydrogen ions repel each other, most equidistant
arrangement is Tetrahedral, 109 degree angles
Same 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
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•
•
•
•
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 Avoidance
Like 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 Illustration
2 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) Symmetry
3 single bonds equivalent (in shape)
to a mix of 2 single + 1 double
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VSEPR Models
• 4-Cloud case
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–
–
–
–
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 Symmetry
4 chlorines at apex of tetrahedron
Similar to Methane (CH4), and many others
cannot 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 eP = 5 e- = 5 eTotal = 40 eThe 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 eF = 7 e- x 4 = 28 eTotal electrons = 34 eWith four atoms and one lone pair, the electron pair geometry is
trigonal 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 Hexafluoride
a 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 carbon
Double bond denies rotation, forces planar shape
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Summary so far …
• Lewis Diagrams, a 2-D structure model
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–
–
–
–
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
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•
•
•
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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 easier
Real molecules hve tetrahedral carbon, zig-zag shaped
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Fig. 13-12, p. 382
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-Octane
Zig-Zag shape from tetrahedral bonding
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“Buckeyballs” named for Buckminster Fuller
60 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 player
http://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 materials
You 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 nomenclature
combine 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
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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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