Unit 10 Flashcards
Linear
-SN=2, 5, 6
-BP=2
-LP=0, 3, 4
-BP-BP=180
Trigonal Planar
-SN=3
-BP=3
-LP=0
-BP-BP=120
Bent
-SN=3
-BP=2
-LP=1
-BP-BP=<120
or
-SN=4
-BP=2
-LP=2
-BP-BP=«109
Tetrahedral
-SN=4
-BP=4
-LP=0
-BP-BP=109
Trigonal Pyramid
-SN=4
-BP=3
-LP=1
-BP-BP=<109
Trigonal Bipyramid
-SN=5
-BP=5
-LP=0
-BP-BP= 120 (eq), 90 (ax)
Seesaw
-SN=5
-BP=4
-LP=1
-BP-BP= <120 (eq), <90 (ax-eq)
T-shape
-SN=5, 6
-BP=3
-LP=2, 3
-BP-BP= <90
Octahedral
-SN=6
-BP=6
-LP=0
-BP-BP=90
Square Pyramid
-SN=6
-BP=5
-LP=1
-BP-BP=<90
Square Planar
-SN=6
-BP=4
-LP=2
-BP-BP=90
VSEPR theory
-Valence Shell Electron-Pair Repulsion
-doesn’t include wave functions
-gives simple rules for predicting molecular shape
-predicts that electron pairs will arrange to minimize repulsion and maximize distance between pairs
3 rules of VSEPR
Steric number rule, repulsion order rule, Bent’s rule
Steric number rule
-SN= number of atoms bonded (BPs) plus number of lone pairs (LPs) around central atom
-determines basic geometry
Repulsion-Order rule
-repulsion is strongest between two lone pairs, followed by repulsion between bond pair and lone pair, followed by repulsion between bond pair and bond pair
-LP/LP > LP/BP > BP/BP
-lone pairs occupy more space, as they are not shared between atoms
Bent’s Rule
-more electronegative atoms prefer axial positions, lone pairs prefer equatorial positions
-electronegativity of atoms surrounding central atom can affect bond angles of molecule
-used for SN=5 and occasionally SN=6
Octet Rule
-atoms gain stability by having 8 electrons in their valence shell
-H only needs 2 surrounding valence electrons
-octet of the central atom should be prioritized
-whenever possible, electrons in Lewis Dot structures are found in pairs
Lewis Dot Structure
-simplified drawings of how valence electrons are arranged around atoms in a molecule
-do not predict shape of molecule
-valid for molecules comprised of “main group” elements (from s and p block)
Guidelines for drawing lewis dot structures
-count up valence electrons contributed by all atoms in molecule
-establish atomic connectivity (central atom will be underlined)
-connect adjacent atoms with electron-pair bonds
-add lone pairs to complete octet on outer atoms
-add remaining electrons
Formal charge
-The charge assigned to an atom in a molecule, assuming that electrons in all chemical bonds are shared equally between atoms, regardless of relative electronegativity
-gives rough estimate of charge distribution in molecules
-FC= number of valence electrons for atom - number of electrons obtained in division
-formal charges of atoms should add up to overall charge on molecule
Rules for obtaining preferred lewis structure
-obey octet rule
-minimize formal charges while preserving octet
-place positive formal charges on more electropositive atoms and negative formal charges on more electronegative atoms
-preferred=lowest energy geometry
Resonance structure
-lewis structures that differ from each other only in the positions of their electrons (atom positions do not change)
-only electrons move, not atoms
-ex. NO3-
Composite structure
how molecule exists in reality, not lewis structure
Bond order
-(number of bonding electron pairs participating in resonance) divided by (number of equivalent bonds)
-count number of bonds in resonance
-ex) for NO3-, BO= 4/3 (1 double bond + 2 single bonds =4…. 3 total equivalent bonds…. 4/3)
-non-integer bond orders are possible when resonance structures occur
Partial charge
-(sum of charges on each equivalent atom) divided by (number of equivalent atoms)
-for one NO3 resonance structure, 3 equivalent O atoms, 2 can have charge of -1 in one structure while other is neutral, PC=-2/3
How bond type affects length/strength
-bond length decreases with bond order (e.g. single(1) is longer than triple(3))
-bond strength increases with bond order (e.g. triple bond(3) is stronger than single(1))
equivalent resonance structures
-multiple Lewis structures of a molecule that have the same stability and charge distribution, contributing equally to the overall resonance hybrid
nonequivalent resonance structures
-different Lewis structures with varying stability, meaning one structure contributes more significantly to the hybrid than the others
Bond Dissociation Energy (BDE)
-an average amount of energy required to break a bond
-on graph, BDE is when slope changes from concave to convex (not bottom of graph)
-BDE increases with higher bond order
