POLAR AND NONPOLAR COMPOUNDS

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.

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 Covalent Bond

A bond between 2 nonmetal atoms that have the same electronegativity and therefore have equal sharing of the bonding electron pair
Example: In H-H each H atom has an electronegativity value of 2.1, therefore the covalent bond between them is considered nonpolar

Polar Covalent Bond

A bond between 2 nonmetal atoms that have different electronegativities and therefore have unequal sharing of the bonding electron pair
Example: In H-Cl, the electronegativity of the Cl atom is 3.0, while that of the H atom is 2.1
The result is a bond where the electron pair is displaced toward the more electronegative atom. This atom then obtains a partial-negative charge while the less electronegative atom has a partial-positive charge.This separation of charge or bond dipole can be illustrated using an arrow with the arrowhead directed toward the more electronegative atom.

The Greek letter delta indicates "partially".

Within a molecule each polar bond has a bond dipole
A polar molecule always contains polar bonds, but some molecules with polar bonds are nonpolar.

Polar Molecule

A molecule in which the bond dipoles present do not cancel each other out and thus results in a molecular dipole.(see below). Cancellation depends on the shape of the molecule or Stereochemistry and the orientation of the polar bonds.

Molecular Dipole

A result of the bond dipoles in a molecule.
Bond dipoles may or may not cancel out thereby producing either molecules that are nonpolar, if they cancel, or polar, if they do not cancel
Examples:

CO₂ is a linear molecule with 2 bond dipoles that are equal and oppositely directed therefore the bond polarities cancel and the molecule is nonpolar.

HCN is a linear molecule with 2 bond dipoles that are in the same direction and are not equal therefore the bond polarities do not cancel and the molecule is polar

More examples can be found on the Table: Stereochemistry of Some Common Molecules

STEREOCHEMISTRY OF SOME COMMON MOLECULES
Compound	Lewis Diagram	# of Lone Pairs Around Central Atom	# of Bonding Electron Groups Around Central Atom	Name of Shape	*Shape Diagram and Bond Dipoles	Polar
C₂H₂	H : C ::: C : H	0	2	linear		no
C₂H₄		0	3	trigonal planar		no
CH₄		0	4	tetrahedral		no
NH₃		1	3	pyramidal		yes
H₂O		2	2	v-shaped (bent)		yes

*NOTE:

For the shape diagrams:

Solid lines represent bonds that are in the same plane as that of the page
Dashed lines represent bonds that are directed into the plane of the page
A wedge indicates bonds that are directed out of the plane of the page

When determining the shapes of molecules the electron pairs of a multiple bond count as a group since the bonds formed are all in the same direction

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