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.