Lewis Structure: Sulfate Ion

Video by Janet Gray Coonce MS

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Now let’s draw the Lewis dot structure for SO₄^-2.  That means we will have to add 2 electrons into our Lewis dot structure.  So we have a sulfur atom with 4 oxygen atoms surrounding it.  Oxygen is in period 2 and sulfur is in period 3.  Sulfur is directly below oxygen on the periodic table and they are both in group 6A.  Both sulfur and oxygen have 6 valence electrons. 

We begin drawing the Lewis dot structure by drawing the 6 valence electrons around each sulfur and oxygen atom.

In this step we added an extra electron to each of the 2 oxygen atoms as illustrated.  Now all of the electrons are accounted for and 2 of the oxygen atoms are electro-negative.

In this step we drew a single bond between each atom.  The oxygen atoms on the left and right each have 8 valence electrons and are happy   but rather than forming a double bond, each of them have acquired an extra electron which will satisfy the octet rule but will make the atom electro-negative.  This explains the negative 2 charge of the sulfate ion.  The top and bottom oxygen molecule each have an unpaired electron which is available to join in a bond with sulfur.

Here we draw a bond between the unpaired electrons.  If we count the electrons, each of the oxygen atoms have 8 electrons in their valence shell so the octet rule is satisfied.  Sulfur has 12 electrons.  It can do this.  It is called an expanded octet.  Atoms belonging to period 3 or more may be able to expand the octet.  Let’s re-draw what we have illustrated here.

T

he two delocalized electrons make multiple resonance structures possible for the sulfate ion (SO₄^-2). This is the completed Lewis dot structure for one of them.  All 6 of sulfur’s valence electrons are being shared in bonds with oxygen giving a total of 12 valence electrons, an expanded octet.  The sulfur atom is happy with valence shell complete and its formal charge is neutral. 

Transcription by James C. Gray MD FACOG