Polar and Non-Polar Molecules
by Ron Kurtus (revised 16 September 2015)
Molecules can be grouped as polar or non-polar molecules. Some molecules are in between the two.
The arrangement or geometry of the atoms in some molecules is such that one end of the molecule has a positive electrical charge and the other side has a negative charge. If this is the case, the molecule is called a polar molecule, meaning that it has electrical poles. Otherwise, it is called a non-polar molecule.
Whether molecules are polar or non-polar determines if they will mix to form a solution or that they don't mix well together. Also, polar molecules are water soluble, while non-polar molecules are fat soluble.
Questions you may have include:
- What is a polar molecule?
- What is an example of a non-polar molecule?
- What rule is there to determine if substances will form solutions?
This lesson will answer those questions.
Chemical bonding is the result of either an atom sharing one or more outer orbit electrons with another atom or an atom taking outer orbit electrons from the atom with which it is bonding. Normally, an atom has an even distribution of electrons in the orbits or shells, but if more end up on one side that the other in a molecule, there can be a resulting electrical field in that area.
Water is polar
Water is a polar molecule because of the way the atoms bind in the molecule such that there are excess electrons on the Oxygen side and a lack or excess of positive charges on the Hydrogen side of the molecule.
Water is a polar molecule with positive charges
on one side and negative on the other
Examples of polar molecules
Examples of polar molecules of materials that are gases under standard conditions are:
- Ammonia (NH3)
- Sulfur Dioxide (SO2)
- Hydrogen Sulfide (H2S).
Also, Ethanol is polar, since its oxygen molecule draws electrons towards it due to its high electro-negativity, causing a negative charge around itself.
A non-polar molecule is one that the electrons are distributed more symmetrically and thus does not have an abundance of charges at the opposite sides. The charges all cancel out each other.
The electrical charges in non-polar Carbon Dioxide are evenly distributed
Examples of non-polar liquids
Most hydrocarbon liquids are non-polar molecules. Examples include:
Alkynes are non-polar because they cannot be dissolved in water, as do polar molecules. However, alkynes but do dissolve in other non-polar substances. A rule is that like substances dissolve in like substances.
(See Hydrocarbon Bonding for more information.)
Examples of non-polar gases
Common examples of non-polar gases are the noble or inert gases, including:
- Helium (He)
- Neon (Ne)
- Krypton (Kr)
- Xenon (Xe)
Other non-polar gases include:
- Hydrogen (H2)
- Nitrogen (N2)
- Oxygen (O2)
- Carbon Dioxide (CO2)
- Methane (CH4)
- Ethylene (C2H4)
Since Chloroform is more soluble in fats than in water, it is also classified as non-polar.
Rule for solutions
The rule for determining if a mixture becomes a solution is that polar molecules will mix to form solutions and non-polar molecules will form solutions, but a polar and non-polar combination will not form a solution.
Water is a polar molecule and oil is a non-polar molecule. Thus they won't form a solution. On the other hand, since alcohol is a polar molecule, it will form a solution with water.
The geometry of atoms in polar molecules is such that one end of the molecule has a positive electrical charge and the other side has a negative charge. Non-polar molecules do not have charges at their ends. Mixing molecules of the same polarity usually results in the molecules forming a solution.
Resources and references
Predicting molecular polarity - Explains polar molecules
Nonpolar Molecule - TutorVista.com
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Polar and Non-Polar Molecules