Notes #10 Lewis Structures. Valence Electrons Valence electrons are those electrons in the highest principle energy level (n). Only these outer electrons.

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Presentation transcript:

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. AlS

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 Cl OCl - (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 Cl Note that each atom is surrounded by 8 e - (Each atom has an “octet”) 8 e -

Lewis Structures Three possibilities for central atom: 1) Just enough e -. Each atom gets 8, and H gets 2. H 2 O (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 -. CO 2 (16 e - ) COO Note: All atoms now have octets. 8 e -

Lewis Structures 3) Too many e -. Atom may have “expanded octet” (more than 8). Put extra e - around central atom. XeF 4 (36 e - ) Xe F 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. CC OO HF

Practice Draw the Lewis structure for the sulfite ion, SO 3 2- SO 3 2- (26 e - ) SO O O

Practice Draw the structure for methanol, CH 3 OH CH 3 OH (14 e - ) C H H HHO

Practice Draw the Lewis structure for the nitrate ion, NO 3 - NO 3 - (24 e - ) N O 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

H 2 O 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…

H 2 O Crisis!

We say that oxygen’s atomic orbitals have morphed into hybridized orbitals. One s orbital + three p orbitals = four sp 3 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 BeF 2 ? 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 BF 3 ? (angle, shape, polarity) 120° angle, trigonal planar, nonpolar

Practice Molecular geometry of carbon tetrafluoride, CF 4 (angle, shape, polarity) 109° angles, tetrahedral, nonpolar

Practice A view of the hybrid orbitals and covalent bonds

Practice Geometry of water, H 2 O (angle, shape, polarity) 105° angle, bent, polar

Practice More views of water

Practice Geometry of ammonia, NH 3 107° 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)