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Lewis Structures
• A Lewis structure shows the distribution of outer (valence) electrons in an atom, molecule, or polyatomic ion.
• Unshared electrons are shown as dots, bonds are shown as straight lines
Transparency #1• In H2O and HF, as in most molecules and
polyatomic ions, nonmetal atoms except H are surrounded by eight electrons, an octet.
• In this sense, each atom has a noble gas structure
• Lewis structures are written following a stepwise procedure
A) Rules for writing Lewis Structures (single bonds)
1. Count valence electrons available. Number of valence electrons contributed by a nonmetal atoms is equal to the last digit of its group number in the periodic table (one for H)
Add electrons to take into account negative charge
• OCl- ion: 6 + 7 + 1 = 14 valence e-
• CH3OH molecule: 4 + 4(1) + 6 = 14 valence e-
• SO3-2 ion:
6 + 3(6) + 2 = 26 valence e-
2.
Draw skeleton structure, using single bonds
• Transparency #2 (next slide)
• Note that carbon almost always forms four bonds. Central atom is written first in formula, terminal atoms are most often H, O, or a halogen
3.
Deduct two electrons for each single bond in the skeleton
OCl- ion: 14 – 2 = 12 valence e- leftCH3OH molecule: 14 – 10 = 4 valence e- leftSO3
-2 ion: 26 – 6 = 20 valence e- left
4.
• Distribute these electrons to give each atom a noble gas structure, if possible
• Transparency # 3
Examples of central atom
• POCl3 P is central atom
• SO4-2
S is central atom
• SO3-2
S is central atom
• PO4-3
P is central atom
• SCl2 S is central atom
B. Too few electrons; form multiple bonds
• Structure of NO3- ion?
• Number of valence electrons = ?
• 5 + 18 + 1 = 24• Go to transparency #4
Exceptions to the octet
• When we must exceed the octet, extra electrons go on central atom. (expanded octet)
• ClF3
• XeO3
• ICl4-
• BeCl2
• Consider XeF4, 36 valence electrons. Octet structure uses 32 electrons. Put extra e- around Xe
• Transparency 5• In a few molecules, there are less than
eight electrons around the central atom• Transparency 5
Resonance
• Sometimes there is more than one valid structure for an molecule or ion.
• NO3-
• Transparency 6• True structure is a hybrid of those three
forms• Use double arrows to indicate it is the
“average” of the structures.• It doesn’t switch between them.
Note that…..
• Resonance forms are obtained by moving electrons, not atoms.
• Resonance can be expected when is possible to draw more that one structure that follows the octet rule
VSEPR
• Lewis structures tell us how the atoms are connected to each other.
• They don’t tell us anything about shape.• The shape of a molecule can greatly
affect its properties.• Valence Shell Electron Pair Repulsion
Theory allows us to predict geometry
VSEPR
• Molecules take a shape that puts electron pairs as far away from each other as possible.
• Have to draw the Lewis structure to determine electron pairs.
• bonding• nonbonding lone pair• Lone pair take more space.• Multiple bonds count as one pair.
Atomic Orbitals - Hybridization
• Valence bond theory modified
• In molecules, the orbitals occupied by electron pairs are seldom “pure” s or p orbitals
• Instead, they are “hybrid” orbitals, formed by combining s, p, d orbitals
Formation of hybrid orbital
• s orbital + p orbital two sp hybrid orbitals– Ex. Be in BeF2
• s orbital + two p orbitals three sp2 hybrid orbitals– Ex. B in BF3
• s orbital + three p orbitals four sp3 hybrid orbitals– Ex. C in CH4
Multiple bonds
• The extra electron pairs in a multiple bond are not located in hybrid orbitals
• (One pair in a double bond, two pairs in a triple bond)
• Pg. 186 Example 7.10
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