SCN− Lewis Structure
Thiocyanate ion · The ion behind the blood-red colour used to test for iron(III) in qualitative analysis.
The SCN- Lewis structure has 16 valence electrons - the 1− charge adds one, drawn as one C=S bond and one C=N bond, with four lone pairs in total, all inside square brackets. The central C is sp hybridized and the ion is linear, with a bond angle of 180 degrees. The 1− charge does not sit on one atom. Across the two resonance structures the N and the S carry it in turn, so the real ion is an average of those forms rather than any one of them, and the charge is shared unevenly between atoms that are not equivalent. There are two competing resonance structures.
- 16 valence e−
- linear
- 180° bond angle
- sp
- 1− charge
SCN− molecular geometry
Linear, from 2 electron domains on the central C: 2 bonded atoms (AX2). The electron geometry is linear. Wedges point toward you, hashed wedges away. Every shape in the VSEPR chart →
Calculated properties
| Total valence electrons | 16 |
|---|---|
| Bonding electrons | 8 (4 shared pairs) |
| Nonbonding electrons | 8 (4 lone pairs) |
| Lone pairs on C | 0 |
| Electron domains (steric number) | 2 |
| VSEPR notation | AX2 |
| Electron geometry | linear |
| Molecular geometry | linear |
| Bond angle | 180° |
| Hybridization | sp |
| Charge | 1− |
| Charge distribution | delocalised over the resonance forms |
| Formal charges | N: -1 |
| Resonance structures | 2 |
| Molar mass | 58.078 g/mol |
S Sulfur 6 valence e− C Carbon 4 valence e− N Nitrogen 5 valence e−
How to draw the SCN− Lewis structure
Step through it. The bar shows how much of the 16-electron budget is spent at each stage - it never grows, which is the whole constraint.
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Skeleton. Join every connected pair with one bond. 2 bonds spends 4 electrons.
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Outer octets. Spend electrons as lone pairs on the outer atoms. That uses the entire budget, and the central atom has nothing left over.
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Multiple bonds. The central atom is still short of an octet, so two lone pairs on the outer atoms move in to be shared. The electron count does not change - the same electrons are just counted by both atoms now.
The reasoning at each step
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Count the valence electrons
Add up the outer-shell electrons every atom brings, then account for the charge. A 1- ion has gained one electron, so it is added to the budget. Skip this and every count that follows will be wrong.
6 (S) + 4 (C) + 5 (N) + 1 (1- charge) = 16 valence electrons -
Choose the central atom
Carbon is the central atom - it forms four bonds and is never terminal in these molecules.
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Join everything with single bonds first
Every connection starts as one shared pair. Draw them all before worrying about double bonds - the arithmetic in the next step is what tells you where the double bonds have to go.
2 bonds x 2 = 4 electrons used, 12 left -
Work out how far short you are
Add up what every atom still needs to fill its shell - eight electrons for most atoms, two for hydrogen - minus what its single bonds already give it. Compare that with the electrons you have left. The difference decides everything that follows.
needs 16, has 12, short by 4 -
Turn the shortfall into multiple bonds
Being short means atoms have to share more. Every pair that moves from a lone pair into a bond counts twice - once for each atom - so a shortfall of 4 is covered by 2 extra shared pairs. That is where the double bonds come from.
C=S, C=N -
Fill in the lone pairs
Every electron not in a bond sits as a lone pair on an atom. Each atom takes exactly what it needs to finish its shell - there is no choice left at this point.
S: 2, N: 2 (8 electrons) -
Check the central atom
C ends up with a full octet. Every electron in the budget is now placed.
C: 8 bonding + 0 nonbonding = 8 -
Check the formal charges
Formal charge is valence electrons, minus lone-pair electrons, minus the number of bonds. They are not real charges, and they are not the ion's charge either - but they must add up to it. Adding these up gives 1-, which is the check that the structure is complete.
N: 5 - 4 - 2 = -1 -
Work out the shape
Count the electron domains on C: 2 bonded groups. A double or triple bond still counts as one domain, because it points in one direction. That gives linear electron geometry; ignore the lone pairs and the atoms themselves sit in a linear arrangement.
steric number 2 -> sp -> linear, bond angle 180 -
Decide whether it is polar
The atoms around the centre are not all the same, so the bond dipoles are different sizes and cannot cancel.
dipoles do not cancel -> polar
Resonance structures
More than one drawing is possible, but these are not equivalent: they put the charge on different elements, so they do not contribute equally. SCN- is an average of the 2 structures below, weighted towards the one that puts negative charge on the more electronegative atom.
Where the 1− charge sits
The 1− charge does not sit on one atom. Across the two resonance structures the N and the S carry it in turn, so the real ion is an average of those forms rather than any one of them, and the charge is shared unevenly between atoms that are not equivalent.
People also ask whether SCN− is polar. Strictly it is neither: polar and nonpolar sort neutral molecules by whether their bond dipoles cancel. A 1− ion is pulled towards any nearby charge whatever its shape, which is why its salts dissolve in water regardless of geometry. The table below is still worth reading - it shows how unevenly each bond holds its electrons, which is what decides whether the charge ends up spread or concentrated.
| Bond | Electronegativity difference | Character |
|---|---|---|
| C–N | 0.49 | polar covalent |
| C–S | 0.03 | nonpolar covalent |
Common questions
How many valence electrons does SCN- have?
SCN- has 16 valence electrons. The atoms bring 15 between them, and the 1− charge adds one. 8 of them are in bonds and 8 sit in lone pairs.
Why does SCN- have a 1− charge?
The 1− charge does not sit on one atom. Across the two resonance structures the N and the S carry it in turn, so the real ion is an average of those forms rather than any one of them, and the charge is shared unevenly between atoms that are not equivalent. The brackets around the drawing are part of the answer: they say the charge belongs to the whole ion rather than to whichever atom happens to be nearest.
What is the molecular geometry of SCN-?
SCN- is a linear ion. The central C has 2 bonded groups, a steric number of 2, which gives linear electron geometry and a linear ion.
What is the bond angle in SCN-?
The bond angle in SCN- is 180 degrees. That is the ideal linear angle, and nothing distorts it here.
What is the hybridization of SCN-?
The central C in SCN- is sp hybridized. Steric number 2 means 2 orbitals have to be mixed, which is exactly what sp gives you.
Is SCN- polar or nonpolar?
Neither, strictly. Polar and nonpolar describe neutral molecules, by asking whether their bond dipoles cancel out. SCN- carries a net 1− charge, so it is pulled towards any nearby charge whatever its shape - which is why ionic compounds dissolve in water regardless of geometry. The question the shape does answer is whether the charge is spread evenly: it is not, because the atoms around the centre are not all the same, so the bond dipoles are different sizes and cannot cancel.
Does SCN- have resonance structures?
Yes, two of them - but they are not equivalent. The charge sits on a different element in each, so they do not contribute equally, and the form that puts the negative charge on the more electronegative atom is the larger contributor.