SCN- Lewis structure

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.

Alternative method: Lewis structure of SCN^–

Rough sketch

• 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

Lone pair

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:

Formal charge

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.

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:

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:

Final structure

The final structure of SCN^– features a central carbon atom linked to a sulfur atom and a nitrogen atom. In this configuration, the carbon atom forms a double bond with the sulfur atom and a double bond with the nitrogen atom to achieve a stable octet. Within this layout, the sulfur atom fulfills its octet by maintaining two lone pairs alongside its double bond, while the nitrogen atom also satisfies the octet rule by retaining two lone pairs. This arrangement represents the most stable state for the ion because it results in a formal charge of 0 on the carbon, 0 on the sulfur, and -1 on the nitrogen, which is the most electronegative atom in the sequence. Accordingly, this specific electronic distribution serves as the definitive and most accurate Lewis representation of the thiocyanate ion.

To complete the representation, draw square brackets around the entire Lewis structure and place a “-” or “-1” sign as a superscript outside the upper right bracket. This notation signifies that the negative charge is a property of the whole ion.

Next: ClF₃ Lewis structure

External video

• SCN- Lewis Structure – How to Draw the Lewis Structure for SCN- (Thiocyanate Ion) – YouTube • Wayne Breslyn

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