# S8E2 - Electronegativity and Bond Polarity

What determines whether a bond is ionic or covalent?  To answer this, you must consider the 2 atoms' or ions' differences in electronegativity...

Electronegativity = the ability of an atom to attract electrons to itself.

## Electronegativity Trend

➞ electronegativity values range from 0.7 to 4.0.

➞ Periodic Trend:

To decide if a bond is ionic or covalent, subtract the 2 electronegativity values, and compare the result to the following chart...

### Bond Polarity Example:

ex
:
Bond Polarity. Order the following bonds according to polarity.

H—H  ,  O—H  ,  Cl—H  ,  S—H  ,  F—H  ,  Na—Cl

To do this, you'll need to use your textbook or this Table of Electronegativity Values to look up each atom's electronegativity value (0.7 - 4.0).

Then, subtract and compare using the handwritten chart above...

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When bonds are polar (ionic or polar covalent), they have a "dipole moment" (or bond dipole).

➞ dipole moment  =  a direction of polarity, shown by the following symbol:

Take a look at the image below.  HCl is a polar covalent bond, and NaCl is an ionic bond.  But both are polar, so both have a dipole moment associated with them.

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## Polarity and Dipole Moments

Besides individual bonds, as we have just seen, these two ideas can also be applied to entire molecules.

In other words, besides "individual bond polarities," we also have "overall molecular polarities."  Let's do an example...

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ex:  For each molecule or ionic compound, indicate which ones are overall polar (i.e. they have an overall dipole moment). Also show the direction of the individual bond polarities.

a.  HCl
2.1  3.0  ➞  electronegativity difference:
= 0.9  ➞  polar covalent bond

b.  CH4
2.5  2.1  ➞  electronegativity difference:
= 0.4  ➞  slightly polar covalent bond

c.  H2S
2.1  2.5  ➞  electronegativity difference:
= 0.4  ➞  slightly polar covalent bond

d.  NaF
0.9  4.0  ➞  electronegativity difference:
= 3.1  ➞  ionic bond (very polar)

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The 3rd video blog entry for SECTION 8 - Chemical Bonding, is up next.

Check it out here:

Ion Electron Configurations, and the Relative Size of Atoms, Cations, and Anions.