Lewis Dot Structures: Electron dot structures of compounds Q: What is the formula for water? H2OH2O Q: How many valence electrons does each hydrogen have?1 Q: How many valence electrons does the oxygen have? 6 Q: How many valence electrons are there in each water molecule? #VE = 2H + 1 oxygen = 2(1) + 6 = 8 More examples 1. How many VE in H 2 O 2 ? #VE = 2(1) + 2(6) = 14 H O 2. How many VE in H 2 SO 4 ?#VE = 2(1) (6) = 32 HSO

The ‘Keep ‘em Happy’ approach to Lewis structures: 1. Add up the number of valence electrons present 2. Draw the stick-skeleton of the molecule 3. Satisfy the octet rule for all atoms in the molecule Exceptions: H only needs 2e- and B only needs 6e- 4. Count up the number of electrons present in the Lewis structure - If there aren’t enough e- (the molecule is ‘unhappy’), add the missing electrons to the central atom - If there are too many e- (the molecule is ‘too happy’), take the excess away from the central atom and then form double bonds with the terminal atoms to satisfy the octet.

Examples: SF 4 #VE = 34 S F F F F 3234 This is the only possible structure because Fluorine NEVER forms double bonds SO 2 SOO #VE = Note: 1 stick = 2 electrons in a bond This molecule is too happy, take away an electron pair from the central atom… …but now sulfur is unhappy with only 6 e- Make sulfur happy by using one of the pairs on oxygen to form a double bond. This molecule isn’t ‘happy’ because it doesn’t have enough electrons.

Example: The odor of the compounds outlined below depend upon their 3D shape Recall that the shape of a molecule can play a very important role in determining its properties.

Ex: CH 4 You might think this is the farthest that the hydrogens can get away from each other 90° 109.5° Molecules will adopt whatever shape allows them to minimize the repulsion between electrons in adjacent bonds. But if you think in 3-dimensions, this shape actually causes less repulsion between the bonding pairs of electrons.

A useful model for predicting the shape of molecules is the… Molecules will adopt a shape that is lowest in energy A low energy shape is one that minimizes the valence shell electron pair repulsion (VSEPR) between adjacent atoms (electrons in bonds and in lone pairs repel each other).

The 5 Main Shapes Molecules adopt a geometry that minimizes electron-electron repulsions  this occurs when e- pairs are as far apart as possible. Linear 180° Trigonal planar 120° Tetrahedral 109.5° Trigonal bipyramidal 120°, 180° Octahedral 90°, 180°

2. Determine the “AXE” notation A = central atom X = # atoms bonded to the central atom E = # of lone pairs on central atom Examples: PH 3 AX 3 E H2SH2SAX 2 E 2 Trigonal pyramidal bent Steps to determining molecular geometry: 1.Draw a Lewis structure 3. Determine the geometry using the AXE chart A X X X E

Let’s look at a few examples…

O H H Trigonal planarbent Trigonal pyramidal AX 3 AX 2 E AX 3 EAX 2 E 2

Going from AXE notation to hybridization of central atom: 1. Add up X and E subscripts on AXE notation Ex: AX 5 E = 6  you have to have 6 orbitals to hold 6 “things” 2. Combine orbitals until the superscripts add up to the same amount (start with s, then p, then d; Maximum of s 1, p 3, d 5 ) Ex:s1s1 p3p3 d2d = 6  You have 6 hybrid orbitals A few more examples: AX 3 E  AX 2 E 3  AX 3  AXE  s1p3s1p3 s1p3d1s1p3d1 s1p2s1p2 s1p1s1p1

The Name Game: Covalent Molecules 1.The first element in the formula is also first in the name and retains the name of the element. 2.The ending of the second element is changed to –ide. 3.Prefixes are used to indicate the number of each element in the molecule. Mono-1Hexa-6 Di-2Hepta-7 Tri-3Octa-8 Tetra-4Nona-9 Penta-5Deca-10

Working out the kinks with a few covalent compounds: 1. CO 2 2. N 2 O 4 3. SF 6 4. PCl 5 5. SO 3 6. H 2 S 7. BCl 3 8. NO 9. CF XeF 4 Carbon dioxide Dinitrogen tetroxide Sulfur hexafluoride Phosphorus pentachloride Sulfur trioxide Dihydrogen monosulfide Boron trichloride Nitrogen monoxide Carbon tetrafluoride Xenon tetrafluoride

Now…change it up a bit and write the formula from the name: 1. Sulfur dioxide 2. Diphosphorus tetroxide 3. Nitrogen trichloride 4. Krypton tetrafluoride 5. Dinitrogen trioxide 6. Dihydrogen monoxide 7. Boron tribromide 8. Carbon monoxide 9. Silicon tetraiodide 10. Disulfur tetroxide SO 2 P2O4P2O4 NCl 3 KrF 4 N2O3N2O3 H2OH2O BBr 3 CO Si I 4 S2O4S2O4