Bonding and Molecular Bonding and Molecular Structure: Fundamental Concepts Structure: Fundamental Concepts Valence e- and Bonding Valence e- and Bonding Covalent Covalent Ionic Ionic Resonance & Exceptions to Octet Resonance & Exceptions to Octet Rule Rule Bond Energy & Length Bond Energy & Length Structure, Shape & Polarity of Structure, Shape & Polarity of AP Notes Chapter 8 AP Notes Chapter 8
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Bonding and Molecular Structure: Fundamental Concepts Valence e- and Bonding CovalentIonic Resonance & Exceptions to Octet Rule Bond Energy & Length Structure,
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Bonding and Molecular Structure: Bonding and Molecular Structure: Fundamental ConceptsFundamental Concepts
Valence e- and BondingValence e- and BondingCovalentCovalent
IonicIonic
Resonance & Exceptions to Octet Resonance & Exceptions to Octet RuleRule
Bond Energy & LengthBond Energy & Length
Structure, Shape & Polarity of Structure, Shape & Polarity of CompoundsCompounds
AP Notes Chapter 8AP Notes Chapter 8
What is a Bond?What is a Bond? A force that holds atoms together.A force that holds atoms together. Why?Why? We will look at it in terms of energy.We will look at it in terms of energy. Bond energy the energy required to Bond energy the energy required to
break a bond.break a bond. Why are compounds formed?Why are compounds formed? Because it gives the system the lowest Because it gives the system the lowest
energy.energy.
Covalent compounds?Covalent compounds?
The electrons in each atom are The electrons in each atom are attracted to the nucleus of the other.attracted to the nucleus of the other.
The electrons repel each other,The electrons repel each other, The nuclei repel each other.The nuclei repel each other. The reach a distance with the lowest The reach a distance with the lowest
possible energy.possible energy. The distance between is the bond The distance between is the bond
ElectronegativityElectronegativity is known for almost every elementis known for almost every element Gives us relative electronegativities Gives us relative electronegativities
of all elements.of all elements. Tends to increase left to right.Tends to increase left to right. decreases as you go down a group.decreases as you go down a group. Noble gases aren’t discussed.Noble gases aren’t discussed. Difference in electronegativity Difference in electronegativity
between atoms tells us how polar.between atoms tells us how polar.
Electronegativity difference
Bond Type
Zero
Intermediate
Large
Covalent
Polar Covalent
Ionic
Co
valent C
haracter
decreases
Ion
ic Ch
aracter increases
Dipole MomentsDipole Moments A molecule with a center of negative A molecule with a center of negative
charge and a center of positive charge and a center of positive charge is charge is dipolar dipolar (two poles),(two poles),
or has a or has a dipole moment.dipole moment. Center of charge doesn’t have to be Center of charge doesn’t have to be
on an atom.on an atom. Will line up in the presence of an Will line up in the presence of an
electric field.electric field.
How It is drawnHow It is drawn
H - F+ -
Which Molecules Have Which Molecules Have Them?Them?
Any two atom molecule with a polar Any two atom molecule with a polar bond.bond.
With three or more atoms there are With three or more atoms there are two considerations.two considerations.
There must be a polar bond.There must be a polar bond. Geometry can’t cancel it out.Geometry can’t cancel it out.
Ionic Radii -- Cations Ionic Radii -- Cations
Ionic Radii -- AnionsIonic Radii -- Anions
Molecular Polarity
Vector Sum of Bond Polarities
BrBr Mg
MgBr2
Mg - Br EN = |1.2 - 2.8| = 1.6
Covalent BOND w/much ionic character, BUT
NON-POLAR molecule
LewisStructures
The most important requirement for the
formation of a stable compound is that the atoms achieve noble gas e- configuration
The structure around a The structure around a given atom is determined given atom is determined principally by minimizing principally by minimizing electron-pair repulsionselectron-pair repulsions
VSEPRVSEPRElectron
pairsBond
AnglesUnderlyingShape
2 180° Linear
3 120° Trigonal Planar
4 109.5° Tetrahedral
590° &120°
Trigonal Bipyramidal
6 90° Octagonal
LEWIS LEWIS STRUCTURESSTRUCTURES
1 : draw skeleton of species: draw skeleton of species2 : count e: count e-- in species in species3 : subtract 2 e: subtract 2 e-- for each bond for each bond
in skeletonin skeleton4 : distribute remaining e: distribute remaining e--
Distinguish Between ELECTRONIC
Geometry&
MOLECULARGeometry
CH4
Electronic geometry: tetrahedral
Molecular geometry: tetrahedral
Bond angle = 109.50
H3O+
Electronic geometry: tetrahedral
Molecular geometry: trigonal pyramidal
Bond angle ~ 1070
H2O
Electronic geometry: tetrahedral
Molecular geometry: bent
Bond angle ~ 104.50
NH2-
Electronic geometry: tetrahedral
Molecular geometry: bent
Bond angle ~ 104.50
“Octet Rule” holds for connecting atoms, but may not for the central
ResonanceResonance Sometimes there is more than one Sometimes there is more than one
valid structure for an molecule or ion.valid structure for an molecule or ion. NONO33
-- Use double arrows to indicate it is Use double arrows to indicate it is
the “average” of the structures.the “average” of the structures. It doesn’t switch between them.It doesn’t switch between them. NONO22
-- Localized electron model is based on Localized electron model is based on
pairs of electrons, doesn’t deal with pairs of electrons, doesn’t deal with odd numbers.odd numbers.
EXAMPLESEXAMPLES
Nitrate ionNitrate ion
OzoneOzone
FORMAL CHARGEFORMAL CHARGEthe charge the charge assignedassigned to an atom to an atom in a molecule or polyatomic ion in a molecule or polyatomic ion
FC atom = Family# - [LPE + ½(BE)]
Sum FC’s atoms = ion charge
Closer sum FC’s is to zero more stable
Formal ChargeFormal Charge For molecules and polyatomic For molecules and polyatomic
ions that exceed the octet there ions that exceed the octet there are several different structures.are several different structures.
Use charges on atoms to help Use charges on atoms to help decide which.decide which.
Trying to use the oxidation Trying to use the oxidation numbers to put charges on atoms numbers to put charges on atoms in molecules doesn’t work. in molecules doesn’t work.
Formal ChargeFormal Charge The difference between the number The difference between the number
of valence electrons on the free atom of valence electrons on the free atom and that assigned in the molecule.and that assigned in the molecule.
We count half the electrons in each We count half the electrons in each bond as “belonging” to the atom.bond as “belonging” to the atom.
SOSO44-2-2
Molecules try to achieve as low a Molecules try to achieve as low a formal charge as possible.formal charge as possible.
Negative formal charges should be Negative formal charges should be on electronegative elements.on electronegative elements.
Assignment of eAssignment of e--
1. Lone pairs belong1. Lone pairs belong entirely to atom in entirely to atom in question question
2. Shared e2. Shared e-- are are divideddivided equally between the equally between the two sharing atoms two sharing atoms
The sum of the formal charges of all atoms in a species must equal the overall charge on the species.
Exceptions to the octetExceptions to the octet BHBH33 Be and B often do not achieve octetBe and B often do not achieve octet Have less than and octet, for Have less than and octet, for
electron deficient molecules.electron deficient molecules. SFSF66 Third row and larger elements can Third row and larger elements can
exceed the octetexceed the octet Use 3d orbitals?Use 3d orbitals? II33
--
Exceptions to the octetExceptions to the octet When we must exceed the octet, When we must exceed the octet,
extra electrons go on central atom.extra electrons go on central atom. ClFClF33
XeOXeO33
IClICl44--
BeClBeCl22
If nonequivalent Lewis structures exist, the one(s) that best describe the bonding in the species has...
FAVORED LEWIS FAVORED LEWIS STRUCTURESSTRUCTURES
1. formal charges closest to 1. formal charges closest to zero zero