covalent bonding bonding models for methane, ch 4. models are not reality. each has its own...
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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 154 347
C = C Double 134 614
C C Triple 120 839
C - O Single 143 358
C = O Double 123 745
C - N Single 143 305
C = N Double 138 615
C N Triple 116 891Bonds 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 energy
Forming = exothermic
The Octet Rule
Combinations 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 Molecule
F
F
1s
1s 2s
2s 2p
2p
Each has seven
valence electrons
F F
The Octet Rule: The Diatomic Oxygen Molecule
O
O
1s
1s 2s
2s 2p
2p
Each has six valence
electrons
O O
The Octet Rule: The Diatomic Nitrogen Molecule
N
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 bond
Oxygen (and sulfur) form two covalent bonds
One double bond, or two single bonds
Nitrogen (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
..
....
..
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
..
..
..
Multiple Covalent Bonds:Double bonds
C C
H
H
H
H
Two pairs of shared electrons
C C
H
H
H
H
Ethene
Multiple Covalent Bonds:Triple bonds
C C HH
Three pairs of shared electrons
Ethyne
C C HH
Acetic Acid
HO
O
H
H
H
C C
Two electrons (one bond) per hydrogen
Eight electrons (four bonds) per carbon
Eight electrons (two bonds, two unshared pairs) per oxygen