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formal charge : the charge a bonded atom would have if all the atoms had the same

# of v.e – in = the isolated – atom. # of e – assigned to the atom in the Lewis structure. formal charge. all unshared e – plus half of the bonding e –. formal charge : the charge a bonded atom would have if all the atoms had the same

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formal charge : the charge a bonded atom would have if all the atoms had the same

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  1. # of v.e– in = the isolated – atom # of e– assigned to the atom in the Lewis structure formal charge all unshared e– plus half of the bonding e– formal charge: the charge a bonded atom would have if all the atoms had the same electronegativity; to find it, you must first draw the Lewis structure When several Lewis structures are possible, the most stable is the one in which: (1) the atoms have the smallest formal charges, and (2) the (–) charges reside on the most electronegative atoms

  2. O (N is more EN than S.) N–C–S Find the formal charge on each atom in the following species. [ ] .. .. .. O S 2– SO32– .. .. O– S –O .. .. v.e– 6 6 .. .. .. assigned e– 5 7 26 e– +1 –1 S: +1 O: –1 .. .. – – – .. .. [ ] [ ] [ ] .. .. .. .. N=C=S NCS– .. N–C–S .. .. .. 5 4 6 5 4 6 5 4 6 16 e– 7 4 5 6 4 6 5 4 7 2– 0 1+ 1– 0 0 0 0 1–

  3. [ ] [ ] [ ] .. .. .. .. – – – .. .. O O .. .. .. .. .. O– N =O .. O= N –O O– N –O .. .. .. .. .. .. .. .. .. .. O .. .. resonance structures: the two or more Lewis structures that are equally correct for a species 24 e– e.g., For NO3–... Resonance structures are a blending of two or more Lewis structures. In drawing resonance structures, e–s move. Atoms don’t move.

  4. H H H H C C C C C C C C H H H H C C C C H H H H Aromatic compounds are based on resonance structures of the benzene molecule. In the interest of efficiency, benzene is often represented like this…

  5. F F .. .. .. .. .. .. F= B –F .. .. .. F– B –F .. .. .. .. .. .. .. .. Exceptions to the Octet Rule There are a few cases (other than for H) in which the octet rule is violated. These are: 1. particles with an odd number of valence e– ClO2, NO, NO2 -- e.g., 2. atoms with less than an octet -- e.g., B, Be (24 e–) BF3 -- would require resonance -- F is WAY more EN than B and won’t share extra e– -- inconsistent w/chemical behavior

  6. .. .. .. .. .. .. –Cl –Cl Cl P Expanded valence shells occur only for atoms in periods > 3. .. .. .. .. .. .. .. Cl– Cl– .. .. 3. atoms with more than an octet -- this occurs when an atom gains an expanded valence shell (40 e–) PCl5 -- other e.g., AsF6–, ICl4–, SF4 -- unfilled d orbitals are involved -- large central atom = more space to accept e– pairs fit more easily around central atom -- small exterior atoms =

  7. .. .. .. .. .. .. (g) 2 (g) I–I I . .. .. .. The strengths of a molecule’s covalent bonds are related to the molecule’s stability and the amount of energy required to break the bonds. bond enthalpy: the DH req’d to break 1 mol of a particular bond in a gaseous substance -- always + At room temp., I2 is a silver-purple solid. -- big DH = strong bond -- e.g., DH = +149 kJ/mol

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