How to memorize electronegativity chart?

I need to know how to tell if molecules are polar or not for my chem exam, but they are not providing an electronegativity chart. Just wondering if there is an easier way than point blank memorization? Thanks.

1 Answer
Truong-Son N.
Mar 5, 2018

Don't memorize it... Furthermore, here are some challenges for you.

  1. Why is #"SO"_2# polar but #"CO"_2# nonpolar?
  2. Why is #"CH"_3"Cl"# polar but #"CCl"_4# nonpolar?
  3. Why is #"BF"_3# nonpolar but #"NF"_3# polar?

Maybe the only ones you could memorize, if you felt like it, are the easy ones:

#"H"#: #2.1#
#"Li"#: #1.0#
#"Be"#: #1.5#
#"B"#: #2.0#
#"C"#: #2.5#
#"N"#: #3.0#
#"O"#: #3.5#
#"F"#: #4.0#

Except for #"H"#, the ones listed are nicely spaced in #0.5# intervals, and fluorine is assigned #4.0# (Pauling made his scale that way). But what you really should do is just know the trend.

Electronegativity increases towards the upper-right.

http://www.dummies.com/education/science/chemistry

A rough guideline is that

#Delta"EN" < 0.5# #-># nonpolar

#0.5 < Delta"EN" < 2.0# #-># polar

#Delta"EN" > 2.0# #-># ionic

And you can estimate this just by saying that

  • bonded identical atoms have #Delta"EN" = 0#, nonpolar

  • bonded nonmetals have approximately #0.5 < Delta"EN" < 1.0# (polar).

  • bonded metal+nonmetal has approximately #1.5 < Delta"EN" < 3.0# (very polar covalent to roughly ionic).

Of course, you also have to consider the symmetry of the molecule! Here are some challenge questions...

  1. Why is #"SO"_2# polar but #"CO"_2# nonpolar?
  2. Why is #"CH"_3"Cl"# polar but #"CCl"_4# nonpolar?
  3. Why is #"BF"_3# nonpolar but #"NF"_3# polar?
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