12/11/09 Chemical Bonding Join hands and sing along….. Review • Polar Bonds v. Non-Polar Bonds v. Ionic Bonds – What’s the difference?? • Valence Electrons – What are they? – C: 1s22s22p2 Which electrons in carbon are its valence electrons? • Dot Diagramming Valence Electrons Review • What in the world is electronegativity?? – Describe the trend. – Increases across periods, decreases down groups. • Differences in electronegativity can help us predict how chemicals react and the type of bond present. 1 12/11/09 Review • ΔE = 0, no polar character, considered non-polar covalent • 0 < ΔE < 1.2, considered non-polar covalent • 1.2 < ΔE < 1.6, considered polar covalent • 1.6 < ΔE , considered ionic • All are less than or equal to. Bonding • This is all models to describe what we see. • The atoms do not know we are talking about them. • As a consequence there are exceptions to the rules. • Learn to understand why exceptions exist to understand the models better. Lewis Structures!! • This is a method, but not the only way, for drawing the structure of molecules. – Add up the total number of valence electrons. – Draw a simple skeleton structure (the least EN element usually goes in the center). – Use two valence electrons to form bonds – Add the left over electrons as lone pairs – Make sure the octet rule is followed (mind the exceptions). 2 12/11/09 Practice • CH4 • PCl3 Exceptions Time! • Polyatomic Ions (NO3-) – Don’t forget charges. – Put in brackets when finished. • Resonance: Same atoms, different arrangement of electrons. • Double and Triple Bonds Formal Charge • This is a tool to help figure out if our structure is correct. • Formal Charge = (Valence e-) – (1/2 bonding electrons) – (lone pair electrons) • Should be zero – If not, put negative charges on most electronegative elements. – Sum of formal charge = charge on molecule 3 12/11/09 Exceptions • Remember:: – Sulfur can bond up to six times (12 electrons) – Phosphorus can bond five times (10 electrons) – Nitrogen can bond four times (rare) – Oxygen can triple bond (only in CO), double bond, and single bond (uncommon). Coordinate Compounds • Coordinate Compounds contain coordinate bonds. – In a coordinate bond both electrons in the bond are from only one of the elements. – Example, NH4+ • This happens in polyatomics. • Rare in neutral compounds. 4