Post on 27-Mar-2015
Chapter 9 Notes – Part II
Mr Nelson 2009
Warm Up – ionic, molecular or acid
• HBr Chlorous Acid
• FeS Copper(I) fluoride
• PBr5 dinitrogen dioxide
• K2SO4 nitrogen monoxide
• CCl4 hydroiodic acid
• HNO3 potassium nitrate
• LiCl phosphoric acid
Resonance
Draw the Lewis dot structure for ozone, O3.
Resonance
• But this is at odds with the true, observed structure of ozone, in which…
– …both O-O bonds are the same length.
– …both O-O bonds have the same strength
• One Lewis structure cannot accurately depict a molecule like ozone.
• We use multiple structures, resonance structures, to describe the molecule.
Resonance Example
• HCO2-
• Electrons that form the second C-O bond in the double bonds below do not always sit between that C and that O
• They move among the two oxygens and the carbon.
Exceptions to the Octet Rule
• Three situations with exceptions to the octet rule:
• When Ions or molecules have:
– an odd number of electrons
– less than an octet (B & Al)
– more than eight valence electrons (an expanded octet)
Exceptions!
• Boron and Aluminum are okay with only 6 e- around them– BF3
More Than an Octet
• PCl5 can only exist is if phosphorus has 10 electrons around it.
– Presumably d orbitals in these atoms participate in bonding.
Different Bond Types
• Ionic (extremely polar)– Electrons are transferred
• Covalent– Polar – uneven sharing of electrons– Nonpolar – even sharing of electrons
Nonpolar Covalent Bonds
Polar Covalent Bonds
Ionic Bonds
Electronegativity• Electronegativity is the ability of atoms in a molecule
to attract electrons to themselves.
• On the PT, EN increases:– …from left to right across a row.
• EN decreases– …from the top to bottom of a group (column).
Bond Properties and Electronegativity
Bond properties and electronegativity
• Any bond can be classified by subtracting the EN of the 2 elements involved
• High differences in electronegativity (1.7 – 4) make the bond IONIC
• Low differences (0-1.7) make the bond COVALENT:– Polar covalent – 0.4 – 1.7
– Nonpolar Covalent – 0 – 0.4
Polar Covalent Bonds
• Electrons are not always shared equally in compounds.
• Oxygen pulls harder on the electrons it shares with hydrogen than hydrogen does.
• Oxygen’s end of the molecule has more electron density than the hydrogen end.
Nonpolar, polar, and ionic bonds
• (a) – a nonpolar covalent bond
• (b) – a polar covalent bond
• (c) – an ionic bond
Polar Covalent Bonds
• When two atoms share electrons unequally, a bond dipole results.
Polar Covalent Bonds
The greater the difference in electronegativity, the more polar is the bond.