Polar Molecules

In the past lesson we learned how to determine whether a bond was polar or non-polar and whether it was covalent or ionic. The "polar" sub-types were found only in covalent bonds and the difference lay in how they shared their bonds.

• In a polar bond, the electrons are shared unequally between two atoms. 
• The electrons are pulled closer to the more electronegative atom, giving that atom a slight negative charge and the other atom a slight positive charge. 
• In a non-polar bond, the electrons are shared equally between two atoms. 
• The electrons are not charged meaning the bond has no positive or negative end.  

In the same way that we can differentiate between polar and non-polar bonds, we can classify molecules either as polar or non-polar.   

• A polar molecule has one end with a positive charge and another end with a negative charge 
• This means polar molecules have an overall charge separation
• Polar molecules are also called dipoles (the prefix di- means two) because of its two charged ends 
• A non-polar molecule has neither positive or negatives charges on its ends 
• This means it is not a dipole  

Determining Polarity 

Being polar on non-polar gives a molecule a variety of different properties. If a molecule contains only non-polar bonds, it will be a non-polar molecule. However, a molecule that contains polar bonds is not necessarily a polar molecule. 

To determine whether a molecule is polar, you need to look at more than just the polarity of its bonds. You need to look at the shape of the molecule. 

*The shape of the molecule and the polarity of its bonds together determine whether the molecule is polar or non-polar* 

  
But the shapes of molecules can get quite convoluted and require further learning on our part so to compensate for that we can look at the molecules symmetry. When observing the symmetry of the molecule to determine its polarity a good rule to keep in mind is this:

• Polar molecules are unsymmetrical  
• And molecules can be unsymmetrical in two ways: 
• Different atoms 
• Different numbers of atoms
• Non-polar molecules are symmetrical (usually) 

The symmetry of a molecule is found by drawing the molecule's Lewis dot diagram or bond diagram and inspecting both the vertical and horizontal symmetries. Remember, symmetry is the quality of being made up of exactly similar parts facing each other or around an axis.  

 

Examples:  

Given the following compounds determine if it is a polar or non-polar molecule: 

• NH₃
• Lewis Diagram:  
• 
• # of Lone Pairs Around Central Atom  
• 1
• # of Bonding Electron Groups Around Central Atom  
• 3
• Name of Shape  
• Pyramidal
• Shape Diagram and Bond Dipoles   
• 
• Symmetric? 
• Asymmetric
• Polar? 
• Polar Molecule

• C₂H₂ 
• Lewis Diagram: 
• H : C ::: C : H
• # of Lone Pairs Around Central Atom  
• 0
• # of Bonding Electron Groups Around Central Atom  
• 2 
• Name of Shape 
• Linear 
• Shape Diagram and Bond Dipoles   
• 
• Symmetric? 
• Symmetric
• Polar? 
• Non-Polar Molecule 

Below is a video that provides a more comprehensive explanation on this subject:  

 

Here's a great acronym for this lesson: 

Symmetric 

Non-Polar

Asymmetric

Polar 

*SNAP*

- Simon Sierra