SCN- Lewis structure

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SCN^– (thiocyanate) has one sulfur atom, one carbon atom, and one nitrogen atom.

In the SCN^– Lewis structure, there are two double bonds around the carbon atom, with sulfur and nitrogen atoms attached to it, and on both sulfur and nitrogen atoms, there are two lone pairs.

Also, there is a negative (-1) charge on the nitrogen atom.

Steps

Here’s how you can easily draw the SCN^– Lewis structure step by step:

#1 Draw a rough skeleton structure
#2 Mention lone pairs on the atoms
#3 If needed, mention formal charges on the atoms
#4 Minimize formal charges by converting lone pairs of the atoms, and try to get a stable Lewis structure
#5 Repeat step 4 again if needed, until all charges are minimized

Now, let’s take a closer look at each step mentioned above.

#1 Draw a rough skeleton structure

• First, determine the total number of valence electrons

In the periodic table, sulfur lies in group 16, carbon lies in group 14, and nitrogen lies in group 15.

Hence, sulfur has six valence electrons, carbon has four valence electrons, and nitrogen has five valence electrons.

Since SCN^– has one sulfur atom, one carbon atom, and one nitrogen atom, so…

Valence electrons of one sulfur atom = 6 × 1 = 6
Valence electrons of one carbon atom = 4 × 1 = 4
Valence electrons of one nitrogen atom = 5 × 1 = 5

Now the SCN^– has a negative (-1) charge, so we have to add one more electron.

So the total valence electrons = 6 + 4 + 5 + 1 = 16

Learn how to find: Sulfur valence electrons, Carbon valence electrons, and Nitrogen valence electrons

• Second, find the total electron pairs

We have a total of 16 valence electrons. And when we divide this value by two, we get the value of total electron pairs.

Total electron pairs = total valence electrons ÷ 2

So the total electron pairs = 16 ÷ 2 = 8

• Third, determine the central atom

We have to place the least electronegative atom at the center.

Since carbon is less electronegative than sulfur and nitrogen, assume that the central atom is carbon.

Therefore, place carbon in the center and sulfur and nitrogen on either side.

• And finally, draw the rough sketch

#2 Mention lone pairs on the atoms

Here, we have a total of 8 electron pairs. And two bonds are already marked. So we have to only mark the remaining six electron pairs as lone pairs on the sketch.

Also remember that sulfur is a period 3 element, so it can keep more than 8 electrons in its last shell. And both (carbon and nitrogen) are the period 2 elements, so they can not keep more than 8 electrons in their last shell.

Always start to mark the lone pairs from outside atoms. Here, the outside atoms are sulfur and nitrogen.

So for sulfur and nitrogen, there are three lone pairs, and for carbon, there is zero lone pair because all six electron pairs are over.

Mark the lone pairs on the sketch as follows:

#3 If needed, mention formal charges on the atoms

Use the following formula to calculate the formal charges on atoms:

Formal charge = valence electrons – nonbonding electrons – ½ bonding electrons

For sulfur atom, formal charge = 6 – 6 – ½ (2) = -1

For carbon atom, formal charge = 4 – 0 – ½ (4) = +2

For nitrogen atom, formal charge = 5 – 6 – ½ (2) = -2

Here, all three atoms have charges, so mark them on the sketch as follows:

The above structure is not a stable Lewis structure because all three atoms have charges. Therefore, reduce the charges (as below) by converting lone pairs to bonds.

#4 Minimize formal charges by converting lone pairs of the atoms

Here we have to convert a lone pair of the sulfur atom (instead of the nitrogen atom). Because sulfur is less electronegative than nitrogen. And so, it can give more valence electrons to share them.

Or remember this way, in SCN^– and similar type structures, we have to make sure that the negative charge comes on the most electronegative element. In this case, the most electronegative element is nitrogen.

Therefore, convert a lone pair of the sulfur atom to make a new C — S bond with the carbon atom as follows:

#5 Since there are charges on atoms, repeat step 4 again

Since there are charges on carbon and nitrogen atoms, again convert a lone pair of the nitrogen atom to make a new C — N bond with the carbon atom as follows:

In the above structure, you can see that the central atom (carbon) forms an octet. And the outside atoms (sulfur and nitrogen) also form an octet. Hence, the octet rule is satisfied.

Now there is still a negative (-1) charge on the nitrogen atom.

This is okay, because the structure with a negative charge on the most electronegative atom is the best Lewis structure. And in this case, the most electronegative element is nitrogen.

Also, the above structure is more stable than the previous structures. Therefore, this structure is the most stable Lewis structure of SCN^–.

And since the SCN^– has a negative (-1) charge, mention that charge on the Lewis structure by drawing brackets as follows:

Next: ClF₃ Lewis structure

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External links

• https://geometryofmolecules.com/scn-lewis-structure-molecular-geometry-hybridization-and-shape/
• https://www.chemistryscl.com/general/lewis-resonance-structures-of-SCN-/
• https://lambdageeks.com/scn-lewis-structure/
• https://www.thegeoexchange.org/chemistry/bonding/Lewis-Structures/SCN-lewis-structure.html