What is polarity?  Focus on covalent bonds  Contributes to the properties of chemical compounds  Based on electronegativity difference between atoms within chemical bonds

Chose your polarity 1) Nonpolar Covalent  small to same electronegativity difference  Occurs among same or similar atoms bonded together  EQUAL sharing of electrons, equal distribution of electron density 2) Polar Covalent  difference in electronegativity  Most electronegative atom pulls electrons towards it  UNEQUAL sharing of electrons, electron density displaced toward electronegative atom

Polar Covalent Bonds

Example 1:  Arrange the following bonds in order of DECREASING polarity.  Br—ClH--Cl  Cl—ClI--Cl  Cl--F

Partial Charges  Exist in polar covalent molecules  Demonstrates tendency of one end to be “slightly” negative or positive  NO charge on the whole molecule

Example 1: H 2 O

Example 2: HCl

Example 3: PH 3

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

Example 1:  Which Lewis structure is more likely???

Resonance  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  True molecule/ion structure ** Draw all resonance structures and connect with double arrow— resonance hybrid is a blend **

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

Example 2:  Draw 3 Lewis structures for NO 3 - and describe how the resonance hybrid relates to all 3.

Homework  Read pp ,  Problems pp , #41, 42, 45, 46, 47, 49, and 51