Depending on the relative electronegativities of the two atoms sharing electrons, there may be partial transfer of electron density from one atom to the other. When the electronegativities are not equal, electrons are not shared equally and partial ionic charges develop.

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The greater the electronegativity difference, the more ionic the bond is. Bonds that are partly ionic are called polar covalent bonds.

Nonpolar covalent bonds, with equal sharing of the bond electrons, arise when the electronegativities of the two atoms are equal.

Polar and Nonpolar Covalent Bonds
Nonpolar CovalentBondA bond between 2 nonmetal atoms that have the sameelectronegativity and therefore have equal sharing of the bonding electronpairExample: In H-H each H atom has an electronegativityvalue of 2.1, therefore the covalent bond between them is considered nonpolarPolar Covalent BondA bond between 2 nonmetal atoms that have differentelectronegativities and therefore have unequal sharing of the bonding electronpairExample: In H-Cl, the electronegativity of the Clatom is 3.0, while that of the H atom is 2.1The result is a bond where the electron pair is displacedtoward the more electronegative atom.

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This atom then obtains a partial-negativecharge while the less electronegative atom has a partial-positive charge.Thisseparation of charge or bond dipole can be illustrated using anarrow with the arrowhead directed toward the more electronegative atom.The Greek letter delta indicates "partially".Within a molecule each polar bond has a bond dipoleA polar molecule always contains polar bonds, butsome molecules with polar bonds are nonpolar.PolarMoleculeA molecule in which the bond dipoles present do notcancel each other out and thus results in a molecular dipole.(seebelow). Cancellation depends on the shape of the molecule or Stereochemistryand the orientation of the polar bonds.Molecular DipoleA result of the bond dipoles in a molecule.Bond dipoles may or may not cancel out thereby producingeither molecules that are nonpolar, if they cancel, or polar, if they donot cancelExamples:CO2 is a linear molecule with 2bond dipoles that are equal and oppositely directed therefore the bondpolarities cancel and the molecule is nonpolar.HCN is a linear molecule with 2 bond dipoles that are in the same directionand are not equal therefore the bond polarities do not cancel and the moleculeis polarMore examples can be found on the Table: Stereochemistryof Some Common MoleculesSTEREOCHEMISTRY OF SOME COMMON MOLECULESCompoundLewisDiagram# of Lone Pairs AroundCentral Atom# of Bonding Electron GroupsAround Central AtomName ofShape*Shape Diagram andBond DipolesPolarC2H2H : C ::: C : H02linearnoC2H403trigonalplanarnoCH404tetrahedralnoNH313pyramidalyesH2O22v-shaped(bent)yes*NOTE:For the shape diagrams:Solid lines represent bonds that are in the sameplane as that of the pageDashed lines represent bonds that are directed intothe plane of the pageA wedge indicates bonds that are directed outof the plane of the pageWhen determining the shapes of molecules the electron pairs of a multiplebond count as a group since the bonds formed are all in the same directionBACK TO MAIN PAAGE