How is the Lewis structure of an ion written?

1 Answer
Truong-Son N.
Jul 31, 2015

The same way you draw the structure of the original molecule or atom, except with #X# more or #X# less electrons, depending on the charge.

i.e. if charge is #-2#, then add #2# electrons (because the charge of an electron is #-1.602xx10^(-19) C# and is thus the same sign as the charge), and vice versa for charges of #pmX#.

So if you use the "counting valence electrons" method, you can draw #SO_4^(2-)# by saying that:

#S: 6#
#O_4: 6*4 = 24#
#"From 2- charge: " 2#

Total: #6+24+2 = color(green)(32)#

Count the electrons in here:
http://chemwiki.ucdavis.edu/
where the above is all of the resonance structures of #SO_4^(2-)#.

With formal charge defined here as #"Actual " e^(-) - "owned " e^-#:

"Owned" electrons:

  • #6# per thionyl oxygen (#S=O#), so #12# total.
  • #7# per oxygen with a formal charge of #-1# (from #6 - 7#), so #14# total.
  • #6# per sulfur, so #6# total.

Sure enough, it matches:
#12+14+6 = color(green)(32)#

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