What happens when a molecule does not seem to have enough valence electrons?  Main goal is to satisfy octet rule for every atom in chemical compound.

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Presentation transcript:

What happens when a molecule does not seem to have enough valence electrons?  Main goal is to satisfy octet rule for every atom in chemical compound  Atom will share more than one electron pair if necessary (create multiple bonds)

Ex. Formaldehyde (H 2 CO)Ex. Formaldehyde (H 2 CO)

Ex. Ozone (O 3 )Ex. Ozone (O 3 )

Formal ChargesFormal Charges  Enables us to determine atoms likely to have a negative or positive charge  Difference between valence electrons in a NEUTRAL atom and valence electrons found in an atom WITHIN A CHEMICAL BOND.  More valence electrons-----negative charge  Less valence electrons------positive charge

Example 1:Example 1:

Formal Charges (cont.)Formal Charges (cont.)  Difference between # of valence electrons present in an atom NOT in chemical bond and # of valence electrons present in an atom WITH a chemical bond  Not actual charges in covalent molecule  Neutral molecules = 0 formal charge  Polyatomic/charged molecule = net charge  Valence Electrons in atom involved in chemical bond  Lone-pair electrons around atom  ½ electrons in a chemical bond

Formal Charges (cont.)Formal Charges (cont.)  Formal Charge = #valence electrons in free atom - #lone pair electrons around bound atom – ½ (# of electrons present in bond with atom)

Example 2:Example 2:  Which Lewis structure is more likely???  Best Lewis Structure for a compound is where atoms have 0 or smallest formal charge and negative formal charges are found in the electronegative atom.

Resonance  Deals with chemical compounds having double/triple bonds.  SOOO more than one Lewis Structure is possible? So which is correct? Neither

Resonance (cont.)Resonance (cont.)  Some molecules or ions have more than one possible Lewis Structure  Structures ONLY vary in electron distribution  Resonance hybrid  A mix/hybrid of all possible Lewis structures—resembles true molecular structure  Same atom arrangement, different electron arrangement so multiple bonds change location in structures ** Draw all resonance structures and connect with double arrow— resonance hybrid is a blend **

Resonance (cont.)Resonance (cont.)  Delocalized Electrons  Electrons are not restricted to where they can “hang out”  Electrons distribute among several atoms  Seen with resonance as electron distribution varies  Localized Electrons  Electron distribution does NOT vary  Electrons stay in a specific region between atoms  No resonance

Example 1: acetate ionExample 1: acetate ion

Example 2:Example 2:  Draw 3 Lewis structures for SO 3 and describe how the resonance hybrid relates to all 3.

Example 3:Example 3:  Draw 3 Lewis structures for NO 3 - and describe how the resonance hybrid relates to all 3.

Homework  Read pp  P. 381 #45-46