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• Organic compounds are compounds containing carbon
• Carbon neither readily gives up nor readily accepts electrons
• Carbon shares electrons with other carbon atoms as well as with several different kinds of atoms
CH1Bonding and Isomerism
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How Electrons are arranged in AtomThe Structure of an Atom
• An atom consists of electrons, positively charged protons,and neutral neutrons
• Electrons form chemical bonds
• Atomic number: numbers of protons in its nucleus
• Mass number: the sum of the protons and neutrons of an atom
• Isotopes have the same atomic number but different mass numbers
• The atomic weight: the average weighted mass of its atoms
• Molecular weight: the sum of the atomic weights of all the atomsin the molecule
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How Electrons are arranged in AtomThe ground-state electronic configuration describes the orbitalsoccupied by the atom’s electrons with the lowest energy
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Lewis’s theory: an atom will give up, accept, or share electrons inorder to achieve a filled outer shell or an outer shell that containseight electrons
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Ionic Bonds Are Formed by the Transfer of Electrons
Attractive forces between opposite charges are called electrostatic attractions
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Polar Covalent Bonds
• Sharing of electrons between atoms of different electronegativities: polar covalent bond (e.g., HCl)
• Equal sharing of electrons: nonpolar covalent bond (e.g., H2, C2H6)
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A polar covalent bond has a slight positive charge on one end and a slight negative charge on the other
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Formal Charge
Formal charge = number of valence electrons –(number of lone pair electrons +1/2 number of bonding electrons)
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Resonance
• Only electrons can be moved (usually lone pairs or pi electrons).
• All the bond lengths are the same.• Each oxygen has a -1/3 electrical charge• The real structure is a resonance hybrid.
N
O
OO
_ _
N
O
OO
_
N
O
OO
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Which of the following choices dose not represent a pair of resonance forms?
A) O OH B) O O
C) NH2 NH2 D) H3C C NH2
O
H3C C NH2
O
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Important Bond Numbers
Q Which of the following pairs will have the same valance?
A) H and Cl B) C and O C) N and C D) B and F
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The orbitals used in bond formation determine the bond angles
• Tetrahedral bond angle: 109.5°
• Electron pairs spread themselves into space as far from each other as possible
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Comparison of C-H bonds:
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Molecule Bond Energy (kcal)
Length (pm)
Ethane Csp3-H 100 110
Ethylene Csp2-H 106 108
Acetylene Csp-H 132 106
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Comparison of C-C bonds:
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Molecule Bond Energy (kcal)
Length (pm)
Ethane Csp3-Csp3 90 154
Ethylene Csp2-Csp2 146 133
Acetylene Csp-Csp 200 120
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Summary
• The shorter the bond, the stronger it is
• The greater the electron density in the region of orbital overlap, the stronger is the bond
• The more s character, the shorter and stronger is the bond
• The more s character, the larger is the bond angle
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