ch08 part5 complex lewis structures
TRANSCRIPT
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There are some molecules andpolyatomic ions for which no singleLewis structure consistent with allcharacteristics and bonding informationcan be written.
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Step 1. The total number of valence electrons
is 24, 5 from the nitrogen atom and 6 fromeach O atom, and 1 from the 1 charge.
Write a Lewis structure for NO2.-
3 NO .
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Since the extra electron present results innitrate having a 1 charge, the ion is enclosedin brackets with a charge.
Step 2. The three O atoms are bonded to a
central N atom. Write the skeletal structureand place two electrons between each pair ofatoms.
Write a Lewis structure for NO2.-
3 NO .
O N O: :O : -
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O N OO
Step 4. Distribute the 18 electrons around the
N and O atoms.
Write a Lewis structure for NO2.-
3 NO .
: : : ::: : : :: : :electron deficient
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Write a Lewis structure for NO2.-
3 NO .
Step 5. One pair of electrons is still needed to
give all the N and O atoms a noble gasstructure. Move the unbonded pair ofelectrons from the N atom and place it
between the N and the electron-deficient Oatom, making a double bond.
: : : ::: : : :O N OO: : :
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Write a Lewis structure for NO2.-
3 NO .
Step 5. One pair of electrons is still needed to
give all the N and O atoms a noble gasstructure. Move the unbonded pair ofelectrons from the N atom and place it
between the N and the electron-deficient Oatom, making a double bond.
NO
::
O :: : O :: : :
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A molecule or ion that shows multiplecorrect Lewis structures exhibits resonance.
Write a Lewis structure for NO2.-
3 NO .
Step 5. There are three possible Lewis
structures.
NO
::
O :: : O ::
: :
- :NO
::
O : : O :: : :
-Each Lewis structure is called a resonancestructure.
NO
::
O :: : O :: : :
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Compounds ContainingPolyatomic Ions
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A polyatomic ion is a stable group of atoms
that has either a positive or negative chargeand behaves as a single unit in many chemicalreactions.
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The nitrate ion is a polyatomic ioncomposed of one nitrogen atom and three
oxygen atoms.
N
O :
:O :: : O :: : :
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Na+
It has a charge of 1 One nitrogen and three oxygen atoms
have a total of 23 valence electrons.
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The 1 charge on nitrate adds an additionalvalence electron for a total of 24.
N
O :
:O :: : O :: : :
-
Na+
The additional valence electron comesfrom a sodium atom which becomes asodium ion.
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Sodium nitrate has both ionic and covalentbonds.
N
O :
:O :: : O :: : :
-
Na+
Ionic bonds exist between the sodiumions and the carbonate ions.
covalentbond
covalent
bond
covalent
bond
ionic bond
Covalent bonds are present betweenthe carbon and oxygen atoms withinthe carbonate ion.
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When sodium nitrate is dissolved in water,the ionic bond breaks.
N
O :
:O :: : O :: : :
-
Na+
The sodium ions and nitrate ions separatefrom each other forming separate sodiumand nitrate ions.
N
O :
:O :: : O :: : :
-
Na+
The nitrate ion, which is held together bycovalent bonds, remains as a unit.
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Molecular Shape
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The 3-dimensional arrangement of the atomswithin a molecule is a significant determinantof molecular interactions.
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The Valence ShellElectron Pair (VSEPR)
Model
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The VSEPR model is based on the idea
that electron pairs will repel each otherelectrically and will seek to minimize thisrepulsion.
To accomplish this minimization, theelectron pairs will be arranged as farapart as possible around a central atom.
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BeCl2 is a molecule with only two pairs ofelectrons around beryllium, its centralatom.Its electrons are arranged 180 o apart formaximum separation.
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BF3 is a molecule with three pairs of electronsaround boron, its central atom.
Its electrons are arranged 120 o apart formaximum separation.
This arrangement of atoms is called trigonalplanar.
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Ball and stick models of methane, CH 4, and carbontetrachloride, CCl 4.
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Ammonia, NH 3, has four electron pairsaround nitrogen.
The arrangementof electron pairsaround nitrogen istetrahedral.
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The NH3
moleculeis pyramidal.
NH3 has oneunbonded pairof electrons.
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Water has four electron pairs aroundoxygen.
The arrangementof electron pairsaround oxygen istetrahedral.
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The H 2O moleculeis bent.
H2O has twounbonded pairsof electrons.
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