Polarity and VSEPR

Covalent Bonding SHARING OF ELECTRONS! Atoms attain an octet (also called noble gas electron configurations) by sharing electrons. EXCEPTION TO OCTET? HYDROGEN – ONLY WANTS 2 ELECTRONS

How the Sharing Happens Hybridization (mixing) of the outer valence electron shells for each atom involved For example: Bonds = overlapping hybrid orbitals

Example: Methane (CH 4 )

Electron Sharing Isn’t Always Equal WHY? ELECTRONEGATIVITY DIFFERENCES! RESULT: Partial positive and partial negative charges

Two Types of Covalent Bonds 1.Polar Covalent -Must indicate charge distribution when drawing -2 acceptable methods: 2.Nonpolar Covalent - Equal or balanced charge distribution

Polar Covalent Bonds Example Species: HCl (Hydrogen Chloride) What periodic trend is involved here? Is the sharing of electrons equal or unequal? Which atom wants electrons more, H or Cl?

Nonpolar Covalent Bonds Example Species: What periodic trend is involved here? Is the sharing of electrons in H 2 equal or unequal?

Which is Which? Bond Energy  Polar bonds generally have higher bond energies (energy necessary to separate the two atoms) than the nonpolar bonds of the parent atoms For example: It takes more energy to separate HF than either H 2 or F 2

Class Questions – Higher Bond Energy? HCl or H 2 NaF or F 2

Thank you, Linus Pauling Pauling has devised a scale for electronegativity based on difference in bond energy SEE THE BACK OF YOUR NOTES HANDOUT for these electronegativity values

Electronegativity Difference Define: ΔEN = Electronegativity Difference = X A – X B X A = Element with higher electronegativity X B = Element with lower electronegativity

What do you think? Which bond type has the greatest electronegativity difference and why?

Predicting the Bond Bond TypeElectronegativity Difference Nonpolar0-0.4 Polar Ionic> 1.7

Practice: YOU DO Calculate the ΔEN for a bond in an H 2 O molecule Classify this bond as nonpolar covalent, polar covalent, or ionic Draw the Lewis dot structure for this molecule Symbolize the partial +/- charges Is H 2 O (the entire molecule) polar? Why/Why Not?

Water is a polar molecule

Molecules: Polar or Not? 2 Factors – Polarity of the individual bonds in the molecule – Shape or geometry of the molecule Pathways to the solution 1. If all bonds are nonpolar, then whole molecule is nonpolar, regardless of its shape 2.If bonds are polar, must look at symmetry of molecule to see if polarity of bonds cancels out

Symmetry or Not?

Example #1: Carbon Disulfide POLAR OR NOT?

Example #2: Nitrogen Trichloride POLAR OR NOT?

Determining Molecular Geometry VSEPR = Valence Shell Electron Pair Repulsion – Assume regions of electron density around atom will be as far apart as possible – Regions of electron density: LONE PAIRS and BOND PAIRS – MULIPLE BONDS count as one bond pair

Electronic Structure (Family) Molecular Structure (Geometry) Areas of e - Density # Of Bonds# Of Lone Pairs Bond Angles Molecular Polarity * **

Determing Molecular Shape: Steps 1.Draw the Lewis Structure 2.Count the number of lone pairs and bond pairs on the central atom 3.Obtain the general shape from the table on your notes handout 4.Alter the shape for repulsion between lone pairs of electrons

Example: IF 3 (Iodine Trifluoride)