8_covalentbonding · ppt file · web view2011-12-06 · covalentbonding. bonding models for...
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CovalentBonding
Bonding models for methane, CH4. Models are NOT reality. Each has its own strengths and limitations.
Polar-Covalent bonds
Nonpolar-Covalent bonds
Covalent Bonds
Electrons are unequally shared Electronegativity difference between .3
and 1.7 Electrons are equally shared Electronegativity difference of 0 to 0.3
Covalent Bonding Forces
Electron – electron repulsive forces Proton – proton repulsive forces Electron – proton attractive forces
Bond Length and Energy
Bond Bond type
Bond length (pm)
Bond Energy(kJ/mol)
C - C Single 154 347C = C Double 134 614C C Triple 120 839C - O Single 143 358C = O Double 123 745C - N Single 143 305
C = N Double 138 615C N Triple 116 891
Bonds between elements become shorter and stronger as multiplicity increases.
Bond Energy and Enthalpy
bondsbroken bonds formedH D D
D = Bond energy per mole of bonds
Energy required Energy released
Breaking bonds always requires energy Breaking = endothermic
Forming bonds always releases energyForming = exothermic
The Octet RuleCombinations of elements tend to form so that each atom, by gaining, losing, or sharing electrons, has an octet of electrons in its highest occupied energy level.
Monatomic chlorine Diatomic chlorine
The Octet Rule and Covalent Compounds
Covalent compounds tend to form so that each atom, by sharing electrons, has an octet of electrons in its highest occupied energy level.
Covalent compounds involve atoms of nonmetals only.
The term “molecule” is used exclusively for covalent bonding
The Octet Rule: The Diatomic Fluorine MoleculeF
F
1s
1s 2s
2s 2p
2p
Each has seven
valence electrons
F F
The Octet Rule: The Diatomic Oxygen MoleculeO
O
1s
1s 2s
2s 2p
2p
Each has six valence
electrons
O O
The Octet Rule: The Diatomic Nitrogen MoleculeN
N
1s
1s 2s
2s 2p
2p
Each has five valence
electrons
N N
Lewis structures show how valence electrons are arranged among atoms in a molecule.
Lewis structures Reflect the central idea that stability of a compound relates to noble gas electron configuration.
Shared electrons pairs are covalent bonds and can be represented by two dots (:) or by a single line ( - )
Lewis Structures
Comments About the Octet Rule 2nd row elements C, N, O, F observe the
octet rule (HONC rule as well). 2nd row elements B and Be often have
fewer than 8 electrons around themselves - they are very reactive.
3rd row and heavier elements CAN exceed the octet rule using empty valence d orbitals.
When writing Lewis structures, satisfy octets first, then place electrons around elements having available d orbitals.
Show how valence electrons are arranged among atoms in a molecule.Reflect the central idea that stability of a compound relates to noble gas electron configuration.
Lewis Structures
The HONC RuleHydrogen (and Halogens) form one covalent bondOxygen (and sulfur) form two covalent bonds
One double bond, or two single bondsNitrogen (and phosphorus) form three covalent bonds
One triple bond, or three single bonds, or one double bond and a single bond
Carbon (and silicon) form four covalent bonds.
Two double bonds, or four single bonds, or a triple and a single, or a double and two singles
CH
H
H
Cl
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Completing a Lewis Structure -CH3Cl
Add up available valence electrons: C = 4, H = (3)(1), Cl = 7 Total = 14
Join peripheral atoms
to the central atom with electron pairs.
Complete octets on
atoms other than hydrogen with remaining electrons
Make the atom wanting the most bonds central
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