CHEM 0310 Lecture 1: Chapter 1

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6 Feb 2017
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Chapter 1: Structure and Bonding in Organic Molecules
Coulombic Attraction
-The nuclei in a compound are held together by the electrons located between them
-Coulo’s la: opposite hages attat eah othe ith a foe iesely popotioal to the suae of
the distance between the centers of the charges
-All chemical bonding involves the electrostatic attraction between the nuclei of atoms and the
neighboring electrons
-Attracting force = 

-The attractive forces between the neutral atoms causes
energy to be released (bond strength)
-When the atoms reach a certain closeness, no more energy
is released and the distance between them now is called
bond length; after this distance is reached, energy increases
-In covalent bonds: when the nuclei are the appropriate
bond length apart, the electrons are spread out around
both nuclei and the attractive/repulsive forces balance for
maximum bonding
-In ionic bonding: alternative mode of bonding that deals
with complete electron transfer and results in two ions
whose opposite charges attract each other
Ionic/Covalent Bonding
-Covalent bonds are formed by the sharing of electrons while an ionic bond is based on the electrostatic
attraction of two ions with opposite charges
-Atoms tend to form molecules in such a way as to reach an octet in the outer electron shell and obtain
noble gas configuration
-In pure ionic bonding, both reaction bonding partners attain noble gas character with their e- transfers
-The hydrogen atom is unique because it may either lose or accept an electron (proton or hydride ion)
-Covalent bonding shares with single (2e), double (4e), and triple (6e) bonds to gain noble gas
configuration
-The sharing of electrons and coulombic attraction contribute to the stability of a bond
-In most organic bonds, the electrons are not shared equally and result in a bond that is in between
covalent and ionic bonding: polar covalent
-The separation of opposite charges is called an electric dipole, symbolized by an arrow pointing from
positive to negative
-In symmetric structures, the polarization of the individual bonds may cancel and lead to molecules with
no net polarization (CO2, CCl4)
-The more electronegative atom is partially negatively charges and has the end of the arrow
Lewis Structures
-These structures are important for predicting geometry and polarity
-Determine the total number of valence electrons, draw the structure, and assign formal charges
-Formal charge = (# of valence electrons) (#of lone pair electrons) (1/2 # of bonding electrons)
-In molecules, charge is not localized on one atom but distributed to varying degrees over its
surroundings
Resonance Structures
-Resonance structure is a way of describing delocalized electrons within certain molecules or polyatomic
ions where the bonding cannot be expressed by one single Lewis structure
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