POLARITYPOLARITY

PolarityPolarityA polar molecule has opposite A polar molecule has opposite

charged ends (+ & -)charged ends (+ & -)The polarity of a bond is a function of The polarity of a bond is a function of

the difference in electronegativity the difference in electronegativity between the two bonded atomsbetween the two bonded atoms

Not every polar bond results in a Not every polar bond results in a polar moleculepolar molecule

Polarity is largely determined by Polarity is largely determined by molecular geometrymolecular geometry

electronegativityelectronegativity

The measure of the attraction an The measure of the attraction an atom has for the electrons in a bondatom has for the electrons in a bond

Periodic trend parallels electron Periodic trend parallels electron affinityaffinity

Pauling scale:Pauling scale:Fluorine assigned a value of 4.0Fluorine assigned a value of 4.0All other elements listed relative to (less All other elements listed relative to (less

than) this valuethan) this valueNonmetals Nonmetals 2 2Metals Metals 2 2

electronegativityelectronegativity

Atoms with different Atoms with different electronegativities pull on the electronegativities pull on the bonding electrons differentlybonding electrons differently

This results in an uneven This results in an uneven distribution of the electrons…distribution of the electrons…

……which then results in a polar bond…which then results in a polar bond…

……which may then result in a polar which may then result in a polar molecule.molecule.

Polar moleculesPolar moleculesIf there are If there are only two atomsonly two atoms in the in the

molecule, and they are molecule, and they are different different elements, the molecule is elements, the molecule is polarpolar

If there are If there are only two atomsonly two atoms in the in the molecule, and they are molecule, and they are the samethe same element, the molecule is element, the molecule is nonpolarnonpolar

Polar moleculesPolar moleculesIf there are If there are more thanmore than two atoms two atoms

in the molecule, a bond being in the molecule, a bond being polar may or may not result in the polar may or may not result in the entire molecule being polarentire molecule being polar

The entire geometry of the The entire geometry of the molecule must be consideredmolecule must be considered

Assymetric molecules tend to be Assymetric molecules tend to be polarpolar

Molecular content and Molecular content and polaritypolarity

If all the atoms covalently bonded to If all the atoms covalently bonded to the central atom are the same, and the central atom are the same, and there are no lone pairs on the central there are no lone pairs on the central atom, the molecule will be atom, the molecule will be nonpolarnonpolar

Even if the bonds are polar, the Even if the bonds are polar, the individual “dipoles” will cancel each individual “dipoles” will cancel each other out.other out.

Example: CHExample: CH4, 4, COCO22

Molecular content and Molecular content and polaritypolarity

If all the atoms bonded to the If all the atoms bonded to the central atom are not the same, central atom are not the same, or do not have the same EN, the or do not have the same EN, the molecule will be molecule will be polarpolar

The individual dipoles will not The individual dipoles will not cancel each other out equallycancel each other out equally

Example: CHExample: CH33F, HCNF, HCN

Molecular shape and polarityMolecular shape and polarity

Lone pairs on the central atom Lone pairs on the central atom tend to result in a tend to result in a polar moleculepolar molecule

The lone pair distorts the symmetry of The lone pair distorts the symmetry of the moleculethe molecule

The individual dipoles will not cancel The individual dipoles will not cancel each other out equallyeach other out equally

Example: HExample: H22O, NHO, NH33

Exceptions: linear (XeFExceptions: linear (XeF22) and square ) and square planar (XeFplanar (XeF44) geometries) geometries

Molecular shape and polarityMolecular shape and polarity

Hydrocarbons composed of Hydrocarbons composed of only C and H are always only C and H are always NONPOLARNONPOLARCHCH44, C, C66HH66, C, C1212HH2424, etc…, etc…

““short chain” (1 – 5 carbon) short chain” (1 – 5 carbon) alcoholsalcohols are usually polarare usually polarCHCH33OH, CHOH, CH33CHCH22OH, COH, C33HH77OH, etc…OH, etc…

What if…What if…

The truth: the polarity of a bond The truth: the polarity of a bond is a continuum rather than an is a continuum rather than an either/or situationeither/or situation

Bonds are classified as either Bonds are classified as either covalent (nonpolar), polar covalent (nonpolar), polar covalent, or ioniccovalent, or ionic

A bond with A bond with EN = 1.7 is EN = 1.7 is considered 50% ionic in natureconsidered 50% ionic in nature

Polarity continuumPolarity continuum

Examples…Examples…ClCl22: : EN = 0.0; this is considered EN = 0.0; this is considered

nonpolar covalentnonpolar covalentBoth of the Cl’s share the Both of the Cl’s share the

electrons equallyelectrons equally

Polar bondsPolar bondsIf both bonding atoms have If both bonding atoms have

identical EN’s, the bonds will be identical EN’s, the bonds will be nonpolarnonpolar

Examples of this are the diatomic Examples of this are the diatomic moleculesmoleculesHH22, N, N22, O, O22, F, F22, Cl, Cl22, Br, Br22, I, I22

Because there are only two atoms Because there are only two atoms in the molecule, the nonpolar in the molecule, the nonpolar bond results in the molecule also bond results in the molecule also being nonpolarbeing nonpolar

Polarity continuumPolarity continuum

Examples…Examples…HCl: HCl: EN = 0.9; this is EN = 0.9; this is

considered polar covalentconsidered polar covalentThe Cl has the electrons a The Cl has the electrons a

majority of the timemajority of the timeBut, the H is not considered to But, the H is not considered to

have “lost” its valence electronhave “lost” its valence electron

Polar bondsPolar bondsIf the bonding atoms have If the bonding atoms have

different EN’s, the resulting different EN’s, the resulting covalent bond will be covalent bond will be polarpolar

The bond is called a polar covalent The bond is called a polar covalent bond, and is often referred to as a bond, and is often referred to as a ““dipoledipole””

Examples would include HCl, Examples would include HCl, HF, CO, among othersHF, CO, among others

Polarity continuumPolarity continuum

Examples…Examples…NaCl: NaCl: EN = 2.1; this is considered EN = 2.1; this is considered

ionicionicThe Cl has the electrons the huge The Cl has the electrons the huge

majority of the timemajority of the timeThe Na is considered to have “lost” its The Na is considered to have “lost” its

valence electronvalence electron In an aqueous solution, the Na is In an aqueous solution, the Na is

indeed Naindeed Na++ and the Cl is Cl and the Cl is Cl--

POLARITYPOLARITY