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Explore the essentials of Lewis structures and VSEPR theory in chemistry. Lewis structures illustrate the distribution of valence electrons in atoms, molecules, or polyatomic ions, showcasing how these electrons form bonds. Learn the stepwise procedure for constructing Lewis structures, including counting valence electrons, forming bonds, and ensuring a noble gas configuration for central atoms. VSEPR theory aids in predicting molecular geometry based on electron pair repulsion, crucial for understanding molecular properties. This guide covers exceptions and resonance, enhancing your grasp of chemical structures.
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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
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
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
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-
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
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
4. • Distribute these electrons to give each atom a noble gas structure, if possible
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
Draw Lewis Dot Structure Ethane C2H6 Check answer on page 167
B. Too few electrons; form multiple bonds • Structure of NO3- ion? • Number of valence electrons = ? • 5 + 18 + 1 = 24 • Go to Elmo #4
Exceptions to the octet • When we must exceed the octet, extra electrons go on central atom. (expanded octet) • ClF3 • XeO3 • ICl4- • BeCl2
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
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.
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
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
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.
