Polar and nonpolar molecules are the two broad classes of molecules. Polarity describes the distribution of electrical charge around a molecule. Charge is evenly distributed in a nonpolar molecule, but unevenly distributed in a polar molecule. In other words, a polar molecule has regions of partial charge.
Here are examples of polar and nonpolar molecules, a look at how polarity relates to ionic and covalent bonds, and how you can use polarity to predict which molecules will mix.
- Nonpolar bonds form between two nonmetals with the same electronegativity value.
- Polar bonds form between atoms of elements with different electronegativity values.
- Nonpolar molecules may contain any type of chemical bonds, but the partial charges cancel each other out.
- Polar molecules contain polar covalent or ionic bonds that are arranges so their partial charges do not cancel each other out.
Polar and Nonpolar Chemical Bonds
Understanding and identifying polar and nonpolar chemical bonds makes it easier to understand polar molecules. In a polar bond, one atom has a partial positive electrical charge, while the other atom has a partial negative electrical charge. In other words, a polar bond forms an electric dipole. In a nonpolar bond, atoms share electrons equally so there is no partial positive or negative charge between them. Whether atoms form polar or nonpolar bonds depends on the difference between their electronegativity values.
- Nonpolar bond: Nonpolar bonds form between two atoms with identical electronegativity values. This type of bond is a pure covalent bond. For example, two hydrogen atoms form a nonpolar bond.
- Polar bond: If the electronegativity values between two atoms are close but not the same, the atoms form a polar covalent bond. Polar covalent bonds form between two different nonmetals. For example, hydrogen (electronegativity = 2.1) and chlorine (electronegativity = 3.0) form a polar covalent bond. If the electronegativity values are very different, the atoms form a polar bond called an ionic bond. Ionic bonds form between metals and nonmetals.
The most polar bond is an ionic bond. A polar covalent bond is slightly polar. A pure covalent bond is nonpolar.
A polar molecule has a dipole, where part of the molecule has a partial positive charge and part has a partial negative charge. Diatomic ionic and polar covalent molecules are polar molecules. But, molecules containing more than two atoms can be polar, too. A polar molecule has an asymmetric shape, lone electron pair, or central atom bonded to other atoms with different electronegativity values. Usually, a polar molecule contains ionic or polar covalent bonds. Examples of polar molecules include:
- Water – H2O
- Ammonia – NH3
- Sulfur dioxide – SO2
- Hydrogen sulfide – H2S
- Carbon monoxide – CO
- Ozone – O3
- Hydrofluoric acid – HF (and other molecules with a single H)
- Ethanol – C2H6O (and other alcohols with an OH at one end)
- Sucrose – C12H22O11 (and other sugars with OH groups)
Polar molecules are often hydrophilic and soluble in polar solvents. Polar molecules often have higher melting points than nonpolar molecules with similar molar masses. This is due to intermolecular forces between polar molecules, such as hydrogen bonding.
Nonpolar molecules form either when electrons are equally shared between atoms in a molecule or when the arrangement of electrons in a molecule is symmetrical so that dipole charges cancel each other out. Examples of nonpolar molecules include:
- Any of the noble gases: He, Ne, Ar, Kr, Xe (Although, technically these are atoms and not molecules.)
- Any of the homonuclear diatomic elements: H2, N2, O2, Cl2 (These are truly nonpolar molecules.)
- Carbon dioxide – CO2
- Boron trifluoride – BF3
- Benzene – C6H6
- Carbon tetrachloride – CCl4
- Methane – CH4
- Ethylene – C2H4
- Hydrocarbon liquids, such as gasoline and toluene
- Most organic molecules, with exceptions (like alcohols and sugars)
Nonpolar molecules share some common properties. They tend to be water insoluble at room temperature, hydrophobic, and able to dissolve other nonpolar compounds.
Nonpolar Molecules With Polar Bonds
Polarity depends on the relative electronegativity values between two atoms forming a chemical bond. Two atoms with the same electronegativity values form a covalent bond. Electrons are shared evenly between atoms in a covalent bond, so the bond is nonpolar. Atoms with slightly different electronegativity values form polar covalent bonds. When electronegativity values between atoms are very different, ionic bonds form. Ionic bonds are highly polar.
Often, the polarity of the bonds is the same as the polarity of the molecule. However, there are nonpolar molecules with polar bonds and polar molecules with nonpolar bonds! For example, boron trifluoride is a nonpolar molecule that contains polar covalent bonds. BF3 is a trigonal planar molecule that evenly distributes electrical charge around the molecule, even though the bond between the boron and fluorine atoms is polar. Ozone is an example of a polar molecule made of nonpolar covalent bonds. The chemical bonds between oxygen molecules in O3 are purely covalent because the atoms have identical electronegativity values. However, the ozone molecule has a bent shape (like water) and its electrons don’t spend equal time with all three atoms. The middle atom has a partial positive electrical charge, while the two outer atoms each bear a partial negative charge.
Polarity and Miscibility
You can use polarity to predict whether or not two compounds are miscible (will mix to form a solution). The rule of thumb is that “like dissolves like.” What this means is that polar solvents dissolve polar solutes, while nonpolar solvents dissolve nonpolar solutes. This explains why alcohol and water are completely miscible (both polar) and why oil and water don’t mix (nonpolar with polar).
A compound with an intermediate polarity between one molecule and another can act as a go-between to dissolve a chemical into a solvent when it’s normally insoluble. For example, to mix an ionic or polar compound into an organic nonpolar solvent, you might first dissolve it in ethanol. Ethanol is only slightly polar, but often it’s enough to dissolve the solute. After the polar molecule has dissolved, mix the ethanol solution into a nonpolar organic solvent, such as xylene or benzene.
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