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Electronegativity

Electronegativity

Author: Katie Smith
Description:

To review the concept of electronegativity
To review numerical values for electronegativity
To identify ionic, covalent and polar covalent bonds by calculating differences in electronegativity.

In this packet, you'll work through several steps as you explore the concept of electronegativity, periodic trends in electronegativity, how to evaluate electronegativity numerically, and how to use differences in electronegativity to identify ionic, polar covalent and covalent bonds.

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Tutorial

Learn about electronegativity! What does it mean? How does electronegativity trend across the periodic table?

Put A Number On It

The idea of "more electronegative" or "less electronegative" can be quantified.  Fluorine (the most electronegative) is given the highest value.  Fluorine has an electronegativity of 4.0.  Cesium has the lowest electronegativity, at 0.7, and everyone else falls somewhere in between.  Values for several commonly encountered elements are shown here, beginning with the most electronegative:

F     4.0          O     3.5              Cl    3.0                N    3.0

Br    2.8          C    2.5                I    2.5                  S    2.5          

H     2.1          Be    1.5              Mg   1.2              Li    1.0

Ca 1.0           Na   0.9               K    0.8 

Source: Russo and Silver, Introductory Chemistry. 3rd Ed. Pearson Benjamin Cummings, 2006. p 185.

What's the Difference?

Imagine getting ready to play a game of tug-of-war.  On one side:  You!  On the other side: You!  The game begins, and a miniature version of you is pulling away at each side.  Who is going to win?!?!?

Not surprisingly, this game is going to end in a draw, and at the end of the day, You 1 and You 2 will share the rope evenly. 

Now imagine a molecule of F2.  The bond between F and F consists of two electrons, and the exact location of those electrons can vary.  Are they both close to F?  Or are they over by the other F?  Maybe they are shared evenly between the two Fluorine atoms?  As we saw in the tug-of-war game above, when both sides are evenly matched, the electrons will be shared evenly -- this is called a covalent bond

When the two sides are not evenly matched, we need to take something else into account:

ELECTRONEGATIVITY!

That makes sense, right?  If electronegativity describes an element's ability to pull electrons towards itself, then the more electronegative element will win the "tug-of-war" by pulling those electrons closer to its side of the bond.  The greater the difference in electronegativity, the more unevenly the electrons are shared.

Covalent bonds, Ionic Bonds, and Polar Covalent Bonds

As you can see in the image above, we can think about sharing in a few different ways.

Option 1:  Perfectly even sharing.  Take the examples above, wherein there was a tug-of-war between two identical people, or a bond between two identical atoms.  The electrons are shared perfectly between the two, and you have a covalent bond.

Option 2:  All mine!  In some situations, the electrons are completely transfered from one atom (the less electronegative element) to another atom (of the more electronegative element).  The result is that each atom now has a charge, and is considered an ion.  To be more specific, the two atoms have opposite charges, and are therefore attracted to each other.  This attraction is at the heart of an ionic bond.

Option 3:  Or, you could have something in the middle.  An uneven sharing, where you get some, and I get a little more.  On another day, I might get a little and you might get a lot.  In general, this category covers all variations that fall somewhere between Options 1 and 2.  Unevenly shared electrons constitute a polar covalent bond.

An Important Note!

Stop!

Before we go any further, consult your textbook or your teacher.  There are several different conventions for what counts as an ionic bond and what counts as a polar covalent bond.  The same general ideas apply, but make sure you use the correct cutoff value on homework or exams!

And, Finally...

We'll work through a few examples of how to use the electronegativity of an element to learn something more about a compound or molecule.