Electronegativity and Electronegativity and PolarityPolarity

Electron AffinityElectron Affinity

Measure of the tendency of an atom to Measure of the tendency of an atom to attract electronsattract electrons

Increases as atomic number increases in Increases as atomic number increases in periodperiod

Decreases as you move down a groupDecreases as you move down a group ElectronegativityElectronegativity

• Indicates relative ability of an element’s atoms Indicates relative ability of an element’s atoms to attract electrons in a chemical bondto attract electrons in a chemical bond

ElectronegativityElectronegativity

Not measure quantities…just numerical Not measure quantities…just numerical values assigned by Linus Pauling to values assigned by Linus Pauling to compare the ability of atoms to attract compare the ability of atoms to attract shared electronsshared electrons

Highest electronegative is F (3.98)Highest electronegative is F (3.98) Lowest electronegative is Fr (0.7)Lowest electronegative is Fr (0.7) Noble gases generally do not form Noble gases generally do not form

compoundscompounds Exception: Larger noble gases (Xe, Rn)Exception: Larger noble gases (Xe, Rn) Table on page 263Table on page 263

How we use electronegativityHow we use electronegativity

Chemical bonds are never completely ionic Chemical bonds are never completely ionic or covalent because shared electrons are or covalent because shared electrons are attracted to each other differentlyattracted to each other differently

Unequal sharing of electrons occurs Unequal sharing of electrons occurs because difference of electronegativitiesbecause difference of electronegativities

Differences in electronegativity between Differences in electronegativity between two atoms can be used to predict the two atoms can be used to predict the character and type of chemical bond character and type of chemical bond formed between two atomsformed between two atoms

Identical atoms have EN difference of zeroIdentical atoms have EN difference of zero• Electrons equally sharedElectrons equally shared• Non-Polar Covalent bond (pure)Non-Polar Covalent bond (pure)

Polar Covalent BondsPolar Covalent Bonds• Unequal sharing of electrons between atoms of Unequal sharing of electrons between atoms of

different elements different elements Ionic BondIonic Bond

• Large difference in EN between atoms indicates Large difference in EN between atoms indicates electron was transferred in a bondelectron was transferred in a bond

Increase difference in ENIncrease difference in EN bond becomes bond becomes more IONICmore IONIC

EN difference 1.70 = 50% ionic and 50% EN difference 1.70 = 50% ionic and 50% covalentcovalent

EN difference >1.70 = IONICEN difference >1.70 = IONIC• Exception: two nonmetals bondingException: two nonmetals bonding

POLAR COVALENT BONDSPOLAR COVALENT BONDS Unequal sharing of electrons in a covalent bondUnequal sharing of electrons in a covalent bond Tug-of-warTug-of-war Electrons being shared spend more time around Electrons being shared spend more time around

the EN atom than the other atom in the bondthe EN atom than the other atom in the bond Results in partial chargesResults in partial charges

• δδ+ and + and δδ--• POLARITYPOLARITY

More EN atom in the bond has the partially More EN atom in the bond has the partially negative chargenegative charge

Less EN atom in the bond has the partially Less EN atom in the bond has the partially positive chargepositive charge

DipoleDipole• Resulting polar bond between two different atoms in a Resulting polar bond between two different atoms in a

covalent bondcovalent bond

Molecular PolarityMolecular Polarity

Polar moleculesPolar molecules attracted to attracted to electric fieldelectric field

Non-polar moleculesNon-polar molecules not attracted not attracted to electric fieldto electric field

Symmetrical molecular shapesSymmetrical molecular shapes NON-POLAR (usually)NON-POLAR (usually)

Asymmetrical moleculesAsymmetrical molecules POLAR as POLAR as long as bond type is polarlong as bond type is polar

ExamplesExamples

Polar or non-polar molecules?Polar or non-polar molecules? CClCCl44 HH22OO SClSCl22 HH22SS CFCF44

CSCS22

NHNH33

SolubilitySolubility

Ability of one substance to dissolve in Ability of one substance to dissolve in another substanceanother substance

Physical propertyPhysical property Determined by bond type and shapeDetermined by bond type and shape Polar molecules soluble in polar Polar molecules soluble in polar

substancessubstances Non-polar molecules soluble in non-Non-polar molecules soluble in non-

polar substancespolar substances

Properties of Covalent CompoundsProperties of Covalent Compounds

Salt vs. sugarSalt vs. sugar Difference in properties result in Difference in properties result in

difference of attractive forcesdifference of attractive forces Bond of atoms in covalent molecule Bond of atoms in covalent molecule

is strongis strong Bond between covalent molecules is Bond between covalent molecules is

weakweak Physical properties due to Physical properties due to

intermolecular forces…intermolecular forces…

Intermolecular ForcesIntermolecular Forces Weak attractive forces between individual Weak attractive forces between individual

moleculesmolecules Van der Waals ForcesVan der Waals Forces

• Non-polarNon-polar Dispersion force or induced dipoleDispersion force or induced dipole Very weakVery weak

• PolarPolar Stronger than dispersionStronger than dispersion Dipole-dipole forceDipole-dipole force Positive end of one dipole is attracted to negative Positive end of one dipole is attracted to negative

end of another dipoleend of another dipole• Hydrogen bondHydrogen bond

Strongest intermolecular forceStrongest intermolecular force Formed between hydrogen of one dipole to the Formed between hydrogen of one dipole to the

fluorine, oxygen, or nitrogen of another dipolefluorine, oxygen, or nitrogen of another dipole

Physical propertiesPhysical properties

Low melting/boiling pointsLow melting/boiling points Many molecular substances are Many molecular substances are

gases or vaporize readily at room gases or vaporize readily at room temptemp

Hardness due to weak intermolecular Hardness due to weak intermolecular forces (paraffin)forces (paraffin)

Solids form weak crystal latticeSolids form weak crystal lattice

Covalent Network SolidsCovalent Network Solids Solid in which Solid in which

atoms are atoms are interconnected by interconnected by network of network of covalent bondscovalent bonds

QuartzQuartz DiamondDiamond Network solids are Network solids are

different from different from molecular solidsmolecular solids• BrittleBrittle• Non-conductors of Non-conductors of

heat and electricityheat and electricity• Very HardVery Hard