Online Chemistry Notes: Electronegativity and Polarity

Electronegativity is a word we use to describe how much an atom is pulling electrons.

The Textbook Definition is:
"Electronegativity: The ability of an atom to attract electrons to itself in a covalent bond."

Fluorine is the most electronegative element.

On a periodic table, Electronegativity increases from left to right across a row, and it also increases up a group from bottom to top.

This is an important concept in chemistry. Electronegativity reappears throughout both semesters of General Chemistry and follows you into Organic and Biological Chemistry if you take those courses. (and you should, or at least sit in on a few classes if you don't want to risk your GPA) 

It's important to understand Electronegativity because it allows us to determine if a molecule is polar. 

What does polar mean? think of it as a molecule with a positive side (pole) and a negative side (pole). Just like a magnet.

Molecules are composed of different atoms, and the more electronegative atom in a molecule will be the negative pole.

 It's pulling those electrons in the covalent bond toward itself. And electrons are negative, so the atom becomes partially negative.

 Similarly, the less electronegative atom will be the positive pole since the electrons are moving away from it.

There are exceptions to this rule. If the pulling force from the electrons, known as the dipole moment, cancels each other out due to symmetric molecular geomoetry, then there will be no net dipole moment, and the molecule will be non-polar. 

Think of CO2

"Carbon dioxide 3D ball" by Jynto (talk) - Own workThis chemical image was created with Discovery Studio Visualizer.. Licensed under CC0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Carbon_dioxide_3D_ball.png#mediaviewer/File:Carbon_dioxide_3D_ball.png

We have a carbon in the middle double 

bonded to 2 Oxygen molecules. 

These oxygen molecules are more electronegative than carbon. So in the bonds between oxygen and carbon, the electrons are being pulled toward the oxygen. 

But since the bond angle is 180 degrees, their pulling force (dipole moment) cancels each other. For that reason CO2 is nonpolar.

Let's look at another molecule H2O:

"Water-3D-balls". Licensed under Public Domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Water-3D-balls.png#mediaviewer/File:Water-3D-balls.png

Now the more electronegative atom, Oxygen, is in the center, and the two Hydrogen atoms are angled away due to 2 lone pairs of electrons on the Oxygen atom (not shown in picture).

The electrons in the bonds between Oxygen and Hydrogen will be pulled toward the oxygen making oxygen partially negative. 

Since the electrons are moving away from the Hydrogen atoms, the Hydrogen atoms become partially positive. 

Most importantly, since the molecular geometry is bent, the dipole moments do not cancel, like they do in Carbon Dioxide, and they cause a net dipole moment toward the oxygen. 

This is why Water is polar and carbon Dioxide is not.

Source:
Tro, Nivaldo J., Robert S. Boikess, Joseph H. Bularzik, and William M. Cleaver. "Liquids, Solids, and Intermolecular Forces." Chemistry A Molecular Approach. Upper Saddle River: Prentice Hall, 2008. 466. Print.