notes #10 lewis structures. valence electrons valence electrons are those electrons in the highest...
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Valence Electrons• Valence electrons are those electrons in the
highest principle energy level (n).
• Only these outer electrons are able to react.
• How many valence e- does phosphorous have?
• Argon?
Valence Electrons• The number of valence e- is the same as the
number for the “A” groups.
Write it down on your own chart!!!
Valence Electrons• How many electrons do most atoms want in
their “valence shell”?
• The Octet Rule – most atoms strive for 8 e- in their outer principle energy level.
• H, Li, Be, and B don’t follow the rule. Why might that be?
Lewis Structures• A Lewis Dot Structure is a way to symbolically
represent the valence electrons of atoms.
• G. N. Lewis discovered the
covalent bond.
• He also coined the term
“photon”.
Lewis Structures• 1) Each dot represents one electron.
• 2) Each line represents two shared electrons in a covalent bond.
Lewis Structures• Imagine there is a square around each
element’s symbol.
• Add dots to each side before doubling up.
Al S
Lewis Structures• Draw magnesium’s Lewis dot structure.
• Draw phosphorus's Lewis dot structure.
• Draw krypton’s Lewis dot structure.
Mg P Kr
Lewis Structures• How to draw Lewis structures for molecules:
• OCl- ion (bleach)
• 1) Count the valence electrons for each atom.
• Add e- for anions (-).
• Subtract e- for cations (+).
O ClOCl- (14 e-)
+ 1 e-
Lewis Structures• 2) Draw a skeleton
structure with lines for shared e-.
• 3) Add e- around atoms until all (14) are used and each has 8 e- surrounding.
OCl- (14 e-)
O ClNote that each atom is surrounded by 8 e-
(Each atom has an “octet”)
8 e- 8 e-
Lewis Structures• Three possibilities for central atom:
• 1) Just enough e-. Each atom gets 8, and H gets 2.
H2O (8 e-)
O HH
Lewis Structures• 2) Not enough e-. Move lone pairs (dots) to
make double or triple bonds until all atoms have 8 e-.
CO2 (16 e-)
CO ONote: All atoms now have octets.
8 e- 8 e-
8 e-
Lewis Structures• 3) Too many e-. Atom may have “expanded
octet” (more than 8). Put extra e- around central atom.
XeF4 (36 e-)
XeF
F
F
F We still need 4 more e-!!!
Lewis Structures• Helpful tips:
• C is always a central atom w/ 4 bonds.
• O usually makes 2 bonds.
• H and the halogens are terminal atoms that make one bond.
• The first atom in the formula is usually the central atom.
C C
O O
H F
Practice• Draw the Lewis structure for the nitrate ion,
NO3-
NO3- (24 e-)
NO
OO
But wait! We have 24 e- but N only has 6 e- surrounding it. Whatever shall we do?
H2O Crisis!
• Take a look at water’s e- configuration and orbitals.
• The prediction is that the covalent bonds with hydrogen will be at right angles.
• In reality…
H2O Crisis!
• We say that oxygen’s atomic orbitals have morphed into hybridized orbitals.
• One s orbital + three p orbitals = four sp3 orbitals.
Predicting Shapes• Molecular shapes predicted using VSEPR
theory.
• All e- pair try and get as far away from each other as possible.
• You MUST draw the Lewis structure before the molecular shape.
Predicting Shapes• Predicting the bond angles
• Add up the number of “forces” surrounding central atom.
• Lone pairs and bonds count as one force.
• Double and triple bonds are also one force.
Predicting Shapes• Three Basic Angles.
• 2 Forces: Linear molecule, 180° angles around central atom
• 3 Forces: Trigonal planar, 120° angles
• 4 Forces: Tetrahedral, 109° angles
Predicting Shapes• You just saw 3 possible angles, but they can
make 5 possible molecular shapes.
• Shape based on 3D arrangement of atoms.
• Lone pair e- influence shape, but are not seen.
Predicting Shape• Determining Polarity• Molecules are polar when more electrons are on one
side than the other.
• If symmetrical, usually nonpolar• If bent, unshared e-, or different terminal atoms,
usually polar.
Partial + Partial -
Practice• What is the molecular geometry of BF3? (angle,
shape, polarity)
• 120° angle, trigonal planar, nonpolar
Practice• Molecular geometry of carbon tetrafluoride, CF4
(angle, shape, polarity)
• 109° angles, tetrahedral, nonpolar
Summary• 2 forces: angle is 180°
can only be linear
• 3 forces: angles are 120°
can be trigonal planar or bent (look for lone pairs)
• 4 forces: angles are about 109°
can be tetrahedral, trigonal pyramidal, or bent.
• 5 or 6 forces: angles are 90° or 120°
can be trigonal bipyramidal, see-saw, octahedral, T-shaped, trigonal planar, or square planar (LOOK AT THE SHEET)