Polarity Review

A review of the general chemistry 1 topics of bond and molecular polarity based on electronegativity and symmetry.

Molecular Polarity

Molecular polarity is based on BOTH bond polarity (difference in electronegativity between the bonding atoms causing an unequal sharing of electrons) and molecular geometry (positioning all bonds within a molecule, causing it to be symmetric* or asymmetric**).

*symmetric molecules are always nonpolar

**asymmetric molecules are polar if they contain polar bonds

Determining Polarity by Molecular Shape

Polarity and Properties

Determining the polarity of a molecule may tell you several things:

i. How high (or low) are the melting and/or boiling points of the substance
ii. How easily the liquid phase of the substance evaporates (vapor pressure)
iii. Whether the substance will dissolve in water or another solvent.

Molecular polarity is the source of attractive intermolecular forces (IMFs). These forces are the reason for the binding properties of adhesives and why some animals can climb up walls and glass. The IMFs act like a magnetic force; the IMFs hold molecules together, similar to how the magnetic force holds magnets together. Some molecules exhibit stronger intermolecular forces, just like some magnets have a stronger magnetic force and are therefore held together more tightly.

Polar Molecules

Polar molecules contain nonmetal atoms that share bonding electrons unequally due to differences in the atoms' electronegativity. If the molecule is also asymmetric (unbalanced)*, there is a greater concentration of electron density on one side of the molecule than on the other.

*Polar molecules are asymmetric because they only have one line of symmetry

The molecule will have one side that has a partial negative charge, and the other side will be partially positive. The oppositely charged ends of these molecules are "poles," just like you have with a magnet. These poles give the molecule an overall dipole, causing the entire molecule to be polar. Polar molecules attract each other; the partial positive (δ+) end of one polar molecule is attracted to the other molecule's partial negative (δ-) end. These are the IMFs mentioned earlier.

If the molecule has polar bonds in an asymmetrical arrangement, the molecule as a whole will have a net dipole directed towards the more electronegative atom or atoms in the molecule. The net direction of the electron pull or density towards the more electronegative atom or atoms may be shown visually using an arrow drawn along the sole axis of symmetry, pointing towards the more electronegative atom or atoms.

To determine which side of the molecule will get the partial positive and which will get the partial negative, you need to look up the atoms' electronegativity (EN) values at either end of the line of symmetry. The side with the more EN atoms will be partially negative (δ-), while the side with the more electropositive (opposite of electronegative) atoms will be partially positive (δ+). The line of symmetry has an arrowhead placed on the more EN end and a plus on the more electropositive end, forming the Dipole Moment. This Dipole Moment represents the direction the electrons are pulled towards.

Structural

Formula

Line of symmetry:

An imaginary line that passes through the center of the molecule, dividing it into identical halves (mirror images)

Electronegativities (EN) of the atoms on either end of the line of symmetry.

The arrow indicates the Dipole Moment. The end with atoms with higher EN values has a greater pull on the electrons, so it is δ- charged. The other end is δ+ charged, forming a polar molecule.

Nonpolar Molecules

If the molecule has a symmetric shape, then the electron density is distributed evenly throughout the molecule. When electron density is uniform, the entire molecule is nonpolar, even if it contains polar bonds.

Nonpolar molecules often are nonpolar because they contain only nonpolar bonds. However, a molecule with polar bonds will be nonpolar if it has two or more lines of symmetry. The electronegativity differences along these lines of symmetry are equal, so there is a constant pull on electrons from all sides of the molecule, resulting in a symmetrical electron distribution.

Because there is an equal distribution of electron density throughout the molecule, no net dipole moment forms. Since the molecule lacks oppositely charged ends or poles, attractive forces will be minimal.

Small nonpolar molecules are usually found in the gaseous state at
room temperature. Some examples of nonpolar molecules that exist in the gaseous state at room temperature are methane (CH4), propane (C3H8), and butane (C4H10). As molecules get larger, they contain more electrons and have greater attractive forces. Therefore, larger nonpolar molecules can be liquids or even solids at room temperature. Examples of nonpolar molecules in the liquid state include octane (C8H18) and benzene (C6H6), while p-dichlorobenzene (C6H4Cl2), what mothballs are made of, is a solid at room temperature.