Polar and Non-Polar Molecules


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.

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. There is a mini-quiz near the end of the lesson.


Polar molecules

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

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

Water is a polar molecule with positive charges
on one side and negative on the other

Gases

Examples of polar molecules of materials that are gases under standard conditions are: Ammonia (NH3), Sulfur Dioxide (SO2) and Hydrogen Sulfide (H2S).

Non-polar molecules

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.

Non-polar Carbon Dioxide

The electrical charges in non-polar Carbon Dioxide are evenly distributed

Liquids

Most hydrocarbons are non-polar molecules. Examples include Toluene and Gasoline. (See Hydrocarbon Bonding for more information.)

Gases

Common examples of non-polar gases are the noble or inert gases, including Helium (He), Neon (Ne), Krypton (Kr) and Xenon (Xe). Other non-polar gases include the Hydrogen (H2), Nitrogen (N2), Oxygen (O2), Carbon Dioxide (CO2), Methane (CH4) and Ethylene (C2H4) molecules.

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.

Summary

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.


Mini-quiz to check your understanding



1. Why is H2O a polar molecule?

It will freeze at polar temperatures

Since Hydrogen is polar and so is Oxygen, then the combination is also polar

The Oxygen bonds with the Hydrogen electrons, causing the (+) Hydrogen nucleus to be exposed at one end

2. Why don't the electrons give a non-polar molecule charges at their ends?

Charges from the protons cancel out charges from the electrons

Non-polar molecules don't have ends

Electrons don't have any charge

3. What happens if you mix two liquids, where each consists of polar molecules, and the liquids don't react chemically?

The combination turns to a solid substance

The combination becomes a solution

Such liquids will not mix

If you got all three correct, you are on your way to becoming a Champion in Chemistry. If you had problems, you had better look over the material again.



 

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