Sulfate Lewis Structure (From class)

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Andrew F 2L
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Joined: Sat Aug 17, 2019 12:17 am

Sulfate Lewis Structure (From class)

Postby Andrew F 2L » Sat Oct 26, 2019 4:03 pm

Can someone explain to me how Dr. Lavelle was able to draw the Lewis Structure for Sulfate because I am confused on how he knew when to stop adding double bonds- I also was confused how he factored in the 2- charge of the ion into the lewis structure; are these two connected?
Thank you!

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Joined: Wed Sep 18, 2019 12:19 am

Re: Sulfate Lewis Structure (From class)

Postby AKhanna_3H » Sat Oct 26, 2019 4:25 pm

He started by finding how many valence electrons Sulfur and Oxygen had. Both have 6, and since there are 4 Oxygens present, there are a total of 32 electrons, so your Lewis structure should include 32 electrons. First he drew the diagram with Sulfur in the center and 4 Oxygens each with a single bond to the Sulfur, with the remaining electrons as lone pairs on the Oxygens. While this structure had the correct number of electrons present, he showed us how formal charge factors into this. The formal charge of an atom indicates a gain or loss of electrons while forming a covalent bond. To calculate a formal charge of an atom, take # of valence electrons - (# of lone pair electrons + # of shared electrons/2). He calculated that the formal charge of the Oxygen atoms (they are all the same since they are all single bonded to Sulfur) to be 6 - (6 +2/2), which equals -1. Every oxygen therefore has an overall formal charge of -1, totaling to -4. Since Sulfate has an overall charge of -2, the Sulfur atom must have a formal charge of +2 (-4 +2 = -2). A structure is most stable if as many atoms as possible have formal charges of 0, which is why Dr. Lavelle rearranged the bonds. An Oxygen that is double bonded to Sulfur would have a formal charge of 0: 6 valence electrons - (4 lone pair electrons + 4 shared electrons/2) = 0. If 2 Oxygens are double bonded, Sulfur's formal charge will also change to 0: 6 - (4 + 4/2) = 0. Based on these calculations, Dr. Lavelle, redrew the Lewis structure so 2 Oxygens are double bonded to the Sulfur and the other 2 Oxygens are single bonded to the Sulfur. The Sulfur and the two double bonded Oxygens have formal charges of 0 and the single bonded Oxygens have formal charges of -1, creating an overall charge of -2 which is what Sulfate has. Now, as many atoms as possible have formal charges of 0, which is more favorable overall.

The best way to figure out how to get the most favorable Lewis structure is to move around the bonds, making sure you always have the correct number of valence electrons. Try to get as many atoms as possible to have a formal charge of 0 while maintaining the overall charge of the molecule.

Hope that helps!

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