lesson 15powerpoint

lesson 15powerpoint - RESONANCE NEW S www.dailynews.com THE...

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ELECTRONS FOUND MOVING THROUGHOUT MOLECULES! RESONANCE RESPONSIBLE LEWIS STRUCTURES TO PROVIDE KEY PIECES OF EVIDENCE RESONANCE NEWS www.dailynews.com THE CHEMIST’S NEWSPAPER - Since 1907
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Localized Electron (L.E.) Model This model assumes that a molecule is composed of atoms that share electron pairs by utilizing their atomic orbitals. In this model: (i)lone pairs are electron pairs localized on an atom and thus are not involved in bonding. Lone pair (ii) bonding pairs are electron pairs located in the space between atoms.
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(i) Lewis Structures The Lewis structure of a molecule shows how valence electrons are arranged among atoms in a molecule. Lewis structures display valence electrons only with the use of dots. The LE Model has three parts: Example: Hydrogen forms stable molecules by sharing two electrons, i.e. it follows the duet rule . H . + . H H:H
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More Examples of Lewis Structures Helium, a noble gas, already obeys the duet rule . He: The second-row non-metals carbon through fluorine form molecules by filling their 2s and 2p orbitals and this requires eight electrons. These elements therefore obey the octet rule Noble gases, except helium , already obey the octet rule. .. .. :Ne:
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Steps for Writing Lewis structures of covalent molecules 1. Calculate the total number of valence electrons by adding all the valence electrons for each atom in the molecule. 2. Divide the total number of valence electrons by 2 to find the number of electron pairs in the molecule. 3. Surround the central atom with four electron pairs and use the remaining electron pairs to complete an octet around each of the other atoms ( this step does not apply to hydrogen). 4. If there are not enough electron pairs to provide an octet for each atom, move an electron pair between two atoms that already share an electron pair.
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Hydrogen atoms are always terminal atoms. Central atoms are generally those with the lowest electronegativity. Carbon atoms are always central atoms. When writing Lewis structures……. .
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Example: Write the Lewis structure of CCl 4 Solution: 1. Sum valence electrons of all atoms: 4 7 4 + (7 x 4) = 32 2. Divide the total number of valence electrons by 2: 3. Surround the central atom with four electron pairs 32 = 16 2 Carbon has lowest electronegativity 4. Use the remaining electron pairs to complete an octet around each of the other atoms. 16 - 4 = 12 Completed Lewis structure
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Example : Draw the Lewis structure of H 2 CO C H H O 1. There is a total of (2 x 1) + 4 + 6 = 12 valence electrons. 2. Dividing total by 2:
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lesson 15powerpoint - RESONANCE NEW S www.dailynews.com THE...

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