Notes #10

Lewis Structures

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

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 sulfite ion,

SO32-

SO32- (26 e-)

S OO

O

Practice• Draw the structure for methanol, CH3OH

CH3OH (14 e-)

CH

HH HO

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?

HOMEWORK• CW#23

• H/O Lewis #1-8, 17, 18

Notes #9b

Molecular Geometry

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!

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 Shapes• What is the molecular geometry for BeF2?

• 180°, linear, non-polar

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 -

Predicting Shape• Polar or Nonpolar?

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

Practice• A view of the hybrid orbitals and covalent bonds

Practice• Geometry of water, H2O (angle, shape, polarity)

• 105° angle, bent, polar

Practice• More views of water

Practice• Geometry of ammonia, NH3

• 107° angles, trigonal pyramidal, polar

Tidbits• Expanded Octet: Six e- forces makes an

octahedron.

Tidbits• If two of those forces are lone pairs, you get

square planar.

Tidbits• It’s all broken down on this handout. It TELLS

you the ANSWERS!

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)