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Electronegativity and Bond Polarity

SECTION 8 - Chemical Bonding


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:

Electronegativity Trends

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

How to Determine Bond Type: Ionic or Covalent

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...

Bond Polarity Examples

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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:What is a Dipole Moment?

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.

Compare the Bonding of HCl and NaCl

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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
 
Describe Bond Polarity of HCl

  b.  CH4
    2.5  2.1  ➞  electronegativity difference:
    = 0.4  ➞  slightly polar covalent bondBond Polarity and Overall Polarity of CH4

  c.  H2S
    2.1  2.5  ➞  electronegativity difference:
    = 0.4  ➞  slightly polar covalent bond
 Describe the Bonding in H2S

  d.  NaF
    0.9  4.0  ➞  electronegativity difference:
    = 3.1  ➞  ionic bond (very polar)
 Sodium Fluoride Ionic Bonding, NaF

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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.