chem1000_lecture21-23_Lewis_and_VSEPR

chem1000_lecture21-23_Lewis_and_VSEPR - Sodium, Na Gallium,...

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CHEMISTRY 1000 Topic #2: The Chemical Alphabet Fall 2011 Dr. Susan Lait Gallium, Ga Sodium, Na Forms of Carbon
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2 Molecules and Covalent Bonds Atoms are most stable when they achieve a noble gas  (or  pseudo noble gas)  electron configuration.  Depending on the  element, this may be most easily achieved by gaining electrons,  losing electrons or sharing electrons.  You may have heard this  tendency referred to as the  octet rule : There are three main types of bonding: Ionic Covalent Metallic
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3 Molecules and Covalent Bonds In a covalent bond, valence electrons are shared between two or  more atoms.  These bonding electrons are literally “co-valent”. Covalent bonds form if heat is released when atoms bond: The negative enthalpy of reaction means that the product (H 2 ) is  more stable than the reactants (2 × H) at constant pressure. By convention, chemists typically write the reverse reaction  equation and list the  bond dissociation enthalpy  (i.e. how much  heat is required to break the bond and move the atoms far  enough away from each other that they don’t interact): H + H H H H = - 435 kJ/mol . . H H H + H BD H = + 435 kJ/mol . .
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4 Properties of Covalent Bonds Determine the enthalpy change that accompanies the reaction  below. BD H (kJ/mol) H-H 436 H-C 414 H-O 464 C-C 347 C-O 360 O-O 142 O=O 498 CH 4(g)   +  ½ O 2(g)     CH 3 OH (g)
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5 Molecules and Covalent Bonds To find the optimum bond length, we can plot energy versus  internuclear distance: The lowest energy point on the graph is at the  average bond  length .  Bonds are not rigid.  At any temperature above 0 K,  they vibrate around the equilibrium bond length. 
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6 Properties of Covalent Bonds Bond Polarity Not all bonds are 100% ionic or 100% covalent. Think of a bond as the rope in a tug-of-war: Both atoms want to keep the electrons in the bond so that they  can have a complete octet.   Sometimes, the two competitors have equal strengths (atoms  with equal electronegativity) so the electrons are shared equally  in a  nonpolar   bond . Sometimes, one competitor is stronger (one atom has higher  electronegativity) so the electrons are pulled toward it and are  shared unequally in a  polar bond . Thus, polarity of a bond depends on the relative ability of each  atom to attract electrons to itself (i.e. its  electronegativity,  χ ): ~0.4 ~2.0 0 Electronegativity difference
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7 Table of Electronegativities
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8 Properties of Covalent Bonds Bond Polarity Recall that electronegativity is conceptually a combination of  electron affinity and first ionization energy.  As such, it:
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This note was uploaded on 03/03/2012 for the course CHEM 1000 taught by Professor Marc during the Fall '06 term at Lethbridge College.

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chem1000_lecture21-23_Lewis_and_VSEPR - Sodium, Na Gallium,...

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