Section 8.5 Electronegativity and Polarity Describe how electronegativity is used to determine bond type. electronegativity: the relative ability of an atom to attract electrons in a chemical bond Compare and contrast polar and nonpolar covalent bonds and polar and nonpolar molecules. Generalize about the characteristics of covalently bonded compounds.

Section 8.5 Electronegativity and Polarity (cont.) polar covalent bond A chemical bond’s character is related to each atom’s attraction for the electrons in the bond.

Electron Affinity, Electronegativity, and Bond Character Most bonds are a blend of ionic and covalent characteristics.Most bonds are a blend of ionic and covalent characteristics. Difference in electronegativity determines bond type.Difference in electronegativity determines bond type. Electronegativity Attraction an atom has for a shared pair of electrons. higher e-neg atom   - lower e-neg atom   +

Electron Affinity, Electronegativity, and Bond Character (cont.) This table lists the character and type of chemical bond that forms with differences in electronegativity.

Electron Affinity, Electronegativity, and Bond Character (cont.) Unequal sharing of electrons results in a polar covalent bond. polar covalent bond Bonding is often not clearly ionic or covalent. This graph summarizes the range of chemical bonds between two atoms.

Polar Covalent Bonds Polar covalent bonds form when atoms pull on electrons in a molecule unequally. Electrons spend more time around one atom than another resulting in partial charges at the ends of the bond called a dipole.

Examples: Cl 2 HCl NaCl =0.0 Nonpolar =0.9 Polar =2.1 Ionic

Nonpolar Covalent Bond e - are shared equally e - are shared equally symmetrical e - density symmetrical e - density usually identical atoms usually identical atoms Bond Polarity

++ -- Polar Covalent Bond e - are shared unequally e - are shared unequally asymmetrical e - density asymmetrical e - density results in partial charges (dipole) results in partial charges (dipole)

NonpolarPolarIonic Bond Polarity

Dipole Moment Direction of the polar bond in a molecule. Arrow points toward the more e - neg atom. H Cl ++ --

Determining Molecular Polarity Depends on: dipole moments dipole moments molecular shape molecular shape

Determining Molecular Polarity Nonpolar Molecules Dipole moments are symmetrical and cancel out. Dipole moments are symmetrical and cancel out. Or… Or… Bonds are nonpolar Bonds are nonpolar BF 3 F F F B

Determining Molecular Polarity Polar Molecules Must have polar bonds and… Must have polar bonds and… Dipole moments are asymmetrical and don’t cancel. Dipole moments are asymmetrical and don’t cancel. net dipole moment H2OH2O H H O

CHCl 3 H Cl Determining Molecular Polarity Therefore, polar molecules have... asymmetrical shape (lone pairs) or asymmetrical shape (lone pairs) or asymmetrical atoms asymmetrical atoms net dipole moment

Intermolecular Forces Attractions are weak compared to ionic and covalent bonds between atoms (intra-bonding) = strong Forces of attraction between two molecules (inter-bonding) = weak Three Types of IMF… Hydrogen Bonding – Strongest of the weak Hydrogen Bonding – Strongest of the weak Dipole (polar) Bonding – medium of the weak Dipole (polar) Bonding – medium of the weak Dispersion Forces – weakest of the weak Dispersion Forces – weakest of the weak

Hydrogen Bonding Bonding between hydrogen and Second row elements N, O, & F only.

Dipole-Dipole Attractions Attractions between polar neighboring molecules. Not as strong as H-bonding

Dispersion Forces Are caused by the movements of electrons around the nucleus of an atom. (Nonpolar Molecules only)Are caused by the movements of electrons around the nucleus of an atom. (Nonpolar Molecules only) More electrons you have the stronger the DF.More electrons you have the stronger the DF. Fritz London Dispersion forces increase with the size of the molecules. (number of atoms)

Relative magnitudes of forces Ionic & Covalent bonds (between atoms) Hydrogen bonding (between molecules) Dipole-dipole interactions (between molecules) Dispersion F orces (between molecules) Strongest Weakest

A.A B.B C.C D.D Section 8-5 Section 8.5 Assessment The force between water molecules is what kind of intermolecular force? A.induced dipole B.hydrogen bond C.sigma bond D.partial dipole

A.A B.B C.C D.D Section 8-5 Section 8.5 Assessment What kind of bond occurs within a molecule with unequal sharing of electron pairs? A.ionic bond B.sigma bond C.non-polar covalent bond D.polar covalent bond