# Lewis Diagrams — Formal Charges?

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What is the total number of valence electrons in the Lewis structure of SeCN-?

What is the formal charge on the C atom in the correct or best Lewis structure for SeCN-?

How many lone pairs are in the best Lewis structure of SCN-?

confused on how to do this

• The valence electrons for the structure is a summation of all the valence electrons of the atoms in the molecule.

In this case, Selenium has a valence shell of 6, Carbon has a valence shell of 4, and Nitrogen has a valence shell of 5. Also, given this is an anion with a -1 charge, you add one more electron to that to get a total of 16 electrons.

The formal charge can be found by taking the valence shell of the atom, subtracting 2 for each lone pair and 1 for each bond. For example, if it is connected by a single bond, you subtract one – if it’s connected by a double bond, you subtract 2.

The Lewis structure should look like the Oxygen is to the far left with 3 lone pairs and is bonded by a single bond to the Carbon which has no lone pairs, which is bonded to the Nitrogen via a triple bond which has one lone pair. This should look fairly similar to the structure of hydrogen cyanide.

In this case, the Carbon has one single bond and one triple bond attached to it, so you subtract 1+3 = 4. There are no lone pairs and so nothing needs to be subtracted there. Therefore, the formal charge on carbon is 4-4 = 0.

A good way to check is to make sure that the net formal charge on the molecule is equal to the charge of the molecule, which in this case is -1. In doing that, we see that Nitrogen has a formal charge of 0 (5-3-2=0) and Oxygen has a formal charge of -1 (6-6-1=-1). Therefore, this can give a good indication that you are correct.

In your next example, the total structure is nearly identical to the SeCN-, except instead of Se we have S. The valence shell is the same for S as it is for Se, and so the Lewis structure can be drawn the same way. In this case, we know that S has 3 lone pairs and N has one, meaning that there is a total of 4 lone pairs.

Source(s): Took chemistry

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