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Honors Chemistry

Honors Chemistry. Lewis Structures and VSEPR Theory. Lewis Structures. A Lewis structure shows the distribution of outer (valence) electrons in an atom, molecule, or polyatomic ion. Unshared electrons are shown as dots Bonds are shown as straight lines.

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Honors Chemistry

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  1. Honors Chemistry Lewis Structures and VSEPR Theory

  2. Lewis Structures • A Lewis structure shows the distribution of outer (valence) electrons in an atom, molecule, or polyatomic ion. • Unshared electrons are shown as dots • Bonds are shown as straight lines

  3. In H2O and HF, as in most molecules and polyatomic ions, nonmetal atoms except H are surrounded by eight electrons, an octet. • In this sense, each atom has a noble gas structure • Lewis structures are written following a stepwise procedure

  4. A) Rules for writing Lewis Structures (single bonds) • Count valence electrons available. (use group number for nonmetals!) • Add electrons to take into account negative charge

  5. OCl- ion: 6 + 7 + 1 = 14 valence e- • CH3OH molecule: 4 + 4(1) + 6 = 14 valence e- • SO3-2 ion: 6 + 3(6) + 2 = 26 valence e-

  6. Draw skeleton structure, using single bonds • Note that carbon almost always forms four bonds. Central atom is written first in formula, terminal atoms are most often H, O, or a halogen

  7. 3. Deduct two electrons for each single bond in the skeleton OCl- ion: 14 – 2 = 12 valence e- left CH3OH molecule: 14 – 10 = 4 valence e- left SO3-2 ion: 26 – 6 = 20 valence e- left

  8. 4. • Distribute these electrons to give each atom a noble gas structure, if possible

  9. Examples of central atom • POCl3P is central atom • SO4-2S is central atom • SO3-2S is central atom • PO4-3P is central atom • SCl2 S is central atom

  10. Draw Lewis Dot Structure Ethane C2H6 Check answer on page 167

  11. B. Too few electrons; form multiple bonds • Structure of NO3- ion? • Number of valence electrons = ? • 5 + 18 + 1 = 24 • Go to Elmo #4

  12. Exceptions to the octet • When we must exceed the octet, extra electrons go on central atom. (expanded octet) • ClF3 • XeO3 • ICl4- • BeCl2

  13. Consider XeF4, 36 valence electrons. Octet structure uses 32 electrons. Put extra e- around Xe • ELMO 5 • In a few molecules, there are less than eight electrons around the central atom

  14. Resonance • Sometimes there is more than one valid structure for an molecule or ion. • NO3- • Transparency 6 • True structure is a hybrid of those three forms • Use double arrows to indicate it is the “average” of the structures. • It doesn’t switch between them.

  15. Note that….. • Resonance forms are obtained by moving electrons, not atoms. • Resonance can be expected when is possible to draw more that one structure that follows the octet rule

  16. VSEPR • Lewis structures tell us how the atoms are connected to each other. • They don’t tell us anything about shape. • The shape of a molecule can greatly affect its properties. • Valence Shell Electron Pair Repulsion Theory allows us to predict geometry

  17. VSEPR • Molecules take a shape that puts electron pairs as far away from each other as possible. • Have to draw the Lewis structure to determine electron pairs. • bonding • nonbonding lone pair • Lone pair take more space. • Multiple bonds count as one pair.

  18. Linear

  19. Octahedral

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