Thiocyanate [SCN]- Lewis dot structure, molecular geometry
or shape, electron geometry, bond angle, hybridization,
formal charges, resonance structure
Thiocyanate ion represented by the formula [SCN]– is found in the salts of thiocyanic acid (HSCN).
Thiocyanate salts such as sodium thiocyanate appear as white crystals with a high melting point and are widely used in the chemical manufacturing industry.
In this article, we have covered some essential chemical properties of [SCN]– ion including how to draw its Lewis dot structure. We will also learn interesting facts about the molecular geometry or shape of [SCN]–, its electron geometry, bond angles, hybridization, formal charges, polarity etc.
The Lewis structure of [SCN]– is composed of three different elemental atoms i.e., a carbon (C) atom at the center which is bonded to a sulfur (S) atom, and a nitrogen (N) atom, one on either side. There are a total of two-electron density regions around the central C-atom in the SCN– Lewis structure.
Both the electron density regions are bond pairs which denotes there is no lone pair of electrons on the central S-atom in the SCN– Lewis dot structure.
Drawing the Lewis dot structure of SCN– is not a difficult task at all. So you may grab a paper and pencil and draw this Lewis structure along with us, using the following simple steps.
Steps for drawing the Lewis dot structure of [SCN]–
1. Count the total valence electrons in SCN–
The Lewis dot structure of a molecule is referred to as a simplified representation of all the valence electrons present in it. Therefore, the very first step while drawing the Lewis structure of SCN– is to calculate the total valence electrons present in the concerned elemental atoms.
As SCN– consists of atoms from three different elements i.e., sulfur (S), nitrogen (N), and carbon (C) so you just need to look for these elements in the Periodic Table.
Carbon (C) is present in Group IV A of the Periodic Table so it has a total of 4 valence electrons. Nitrogen (N) is present in Group V A so it has 5 valence electrons while sulfur (S) is situated in Group VI A of the Periodic Table which means it has a total of 6 valence electrons in each atom.
The [SCN]– ion consists of 1 C-atom, 1 N-atom, and 1 S-atom. Thus, the valence electrons in the Lewis dot structure of [SCN]– = 1(4) + 1(5) + 1(6) = 15 valence electrons.
However, the twist here is that the [SCN]– ion carries a negative (-1) charge which means 1 extra valence electron is added in this Lewis structure.
∴ Hence, the total valence electrons available for drawing the Lewis dot structure of [SCN]– = 15 + 1 = 16 valence electrons.
2. Choose the central atom
In the second step of drawing the Lewis structure of a molecule or a molecular ion, we need to place the least electronegative atom at the center.
As electronegativity refers to the ability of an elemental atom to attract a shared pair of electrons from a covalent chemical bond therefore the least electronegative atom is the one that is most likely to share its electrons with other atoms.
As carbon is less electronegative than both nitrogen and sulfur, so, in the Lewis structure of [SCN]–, we will place the C atom at the center while N and S atoms are placed in its surroundings, as shown in the figure below.
3. Connect outer atoms with the central atom
Now we need to connect the outer atoms with the central atom of a Lewis structure using single straight lines. As both nitrogen and sulfur atoms are the outer atoms in [SCN]– Lewis structure while the carbon atom is the central atom, so we will connect N and S atoms with the central C-atom using straight lines, as shown below.
Each straight line represents a single covalent bond i.e., a bond pair containing 2 electrons. There are a total of 2 single bonds in the above diagram which means a total of 2(2) = 4 valence electrons are used till this step, out of the 16 initially available.
Total valence electrons available – electrons used till step 3 = 16-4 = 12 valence electrons.
This means we still have 12 valence electrons to be accommodated in the Lewis dot structure of [SCN]–.
4. Complete the octet of outer atoms
As we already identified, the nitrogen and the sulfur atoms act as outer atoms in SCN– Lewis structure and both N and S atoms need a total of 8 valence electrons to achieve a stable octet electronic configuration.
Single bonds with the central C-atom on each side of the ion show both N and S atoms already have 2 electrons. So, both N and S atoms require 6 more valence electrons to complete their octet.
Thus, these 6 electrons are placed as 3 lone pairs around each outer atom in SCN– Lewis structure, as shown in the figure below.
5. Complete the octet of the central atom and make a covalent bond if necessary
Total valence electrons used till step 4 = 2 single bonds + electrons placed around N-atom + electrons placed around S-atom = 2(2) + 6 + 6 = 16 valence electrons.
Total valence electrons available – electrons used till step 4 = 16-16= 0 valence electrons.
As all the 16 valence electrons initially available are already used up in drawing the Lewis dot structure of SCN– so there is no lone pair on the central C-atom.
But the problem here is that there are only 2 single bonds around the central C-atom which mean there are only 4 valence electrons around it. This denotes that this carbon has an incomplete octet and it still needs 4 more electrons to achieve a stable octet configuration.
We can solve this problem by converting the lone pairs of electrons present on the outer atoms into covalent bonds between the central C-atom and the concerned outer atom.
So, 2 lone pairs of electrons, one from the terminal N atom and another from the terminal S atom are converted into covalent bonds between the central C and the concerned outer atoms, as shown in the diagram below.
In this way, there is a double covalent bond between the central C and each of the other two outer atoms. The central C-atom now has a complete octet (2 double bonds). The octets of the outer N and S atoms are also complete with 1 double bond and 2 lone pairs on each atom.
The final step is to check the stability of the Lewis structure obtained in step 5. Let us do that using the formal charge concept.
6. Check the stability of the SCN– Lewis structure using the formal charge concept
The less the formal charge on the atoms of a molecule, the better the stability of its Lewis structure.
The formal charge can be calculated using the formula given below.
This calculation shows that zero formal charges are present on the carbon and sulfur atoms in SCN– Lewis structure. However, a -1 formal charge is present on the most electronegative nitrogen atom which is also the charge present on the thiocyanate ion, overall.
In accordance with this, the SCN– Lewis structure is enclosed in square brackets and a negative 1 charge is placed at the top right corner, as shown below. This ensures that it is a correct and stable Lewis representation for the thiocyanate [SCN]– ion.
An important point to remember is that the actual Lewis structure of the thiocyanate ion is a hybrid of the following resonance structures.
Each resonance structure is a way of representing the Lewis structure of a molecule or a molecular ion.
The above three resonance structures are not equivalent. The un-bonded and the pi-bonded electrons present in the molecule keep revolving from one position to another. Consequently, the formal charges present on the bonded atoms and the position of covalent bonds change.
Resonance structure (3) is the least stable Lewis representation of [SCN]– due to the concentration of formal charges. In resonance structure (2), there is a -1 charge on the sulfur atom while a triple bond is formed between the central C-atom and the terminal N-atom. This is also a stable [SCN]– Lewis structure.
Alternately, resonance structure (1) is the best possible and most stable Lewis representation of the thiocyanate [SCN]– ion because the -1 charge is present on the more electronegative atom i.e., nitrogen as opposed to the less electronegative sulfur, agreed with what we obtained in step 5.
What are the electron and molecular geometry of SCN-?
The thiocyanate [SCN]– ion has an identical electron pair and molecular geometry or shape i.e., linear. Owing to the absence of any lone pair of electrons on the central C-atom in the [SCN]– molecular ion, there is no distortion present in the shape and geometry of the ion.
Molecular geometry of [SCN]–
Thiocyanate [SCN]– ion displays a linear shape or molecular geometry. Both the nitrogen and sulfur atoms bonded to the central C-atom lie on a straight line, in a planar arrangement. There is no lone pair of electrons on the central carbon atom therefore no bond pair-lone pair and lone pair-lone pair repulsions exist in the molecular ion.
S=C and C=N bond pair-bond pair electronic repulsions exist which push the bond pairs as far apart from each other as possible and the S and N atoms occupy the terminals, as shown in the figure below.
Electron geometry of [SCN]–
According to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, the ideal electronic geometry of a molecule or a molecular ion containing a total of 2 electron density regions around the central atom is linear.
In the thiocyanate [SCN]– ion, there are 2 double bonds around the central carbon atom which makes a total of 2 electron density regions. Thus, its electron geometry is also linear.
An easy way to find the shape and geometry of the molecule is to use the AXN method.
AXN is a simple formula to represent the number of atoms bonded to the central atom in a molecule and the number of lone pairs present on it.
It is used to predict the geometry or shape of a molecule using the VSEPR concept.
AXN notation for [SCN]– molecular ion
A in the AXN formula represents the central atom. In the [SCN]– ion, carbon (C) is present at the center so A = Carbon.
X denotes the atoms bonded to the central atom. In [SCN]–, 1 nitrogen (N) atom and 1 sulfur (S) atom are bonded to the central C so X = 1+1 =2.
N stands for the lone pairs present on the central atom. As per the Lewis structure of [SCN]– there is no lone pair on central carbon so N=0.
Thus, the AXN generic formula for the [SCN]– ion is AX2.
Now, you may have a look at the VSEPR chart below.
The VSEPR chart confirms that the ideal electron geometry and molecular geometry or shape of a molecule with AX2 generic formula are identical i.e., linear, as we already noted down for the [SCN]– ion.
Hybridization of [SCN]–
The thiocyanate [SCN]– ion has sp hybridization.
The electronic configuration of a carbon (C) atom is 1s2 2s2 2p2.
During chemical bonding, the 2s electrons get unpaired and one of the two 2s electrons of carbon shifts to the empty 2p orbital. This results in half-filled 2s and 2p atomic orbitals. Consequently, one 2s and one 2p orbital mix to yield two equivalent sp hybrid orbitals. Each sp hybrid orbital contains a single electron only and possesses a 50% s-character and a 50% p-character.
One sp hybrid orbital of carbon forms the C-S sigma (σ) bond while the other sp hybrid orbital forms the C-N sigma (σ) bond by sp-sp2 overlap with sulfur and nitrogen atoms respectively.
The unhybridized p orbitals of carbon form the required pi (π) bonds by the p-p side-by-side overlap of atomic orbitals, on each side of the molecular ion.
The outer nitrogen and sulfur atoms are sp2 hybridized in SCN– which uses one of the two sp2 hybrid orbitals for the C-N and C-S sigma bonds, as discussed above, while the other sp2 hybrid orbitals contain lone pair of electrons, as shown in the figure below.
But as the hybridization of a molecule or molecular ion is considered with respect to the central atom i.e., C-atom in [SCN]– so it has sp hybridization overall.
A shortcut to finding the hybridization present in a molecule or a molecular ion is by using its steric number against the table given below. The steric number of central C in [SCN]– is 2 so it has sp hybridization.
Steric number
Hybridization
2
sp
3
sp2
4
sp3
5
sp3d
6
sp3d2
The [SCN]– bond angle
As all three bonded atoms (S, C, and N) lie on a straight line in the linear [SCN]– ion therefore they form a mutual bond angle of 180°. The C=S bond length is 167.9 pm while the C=N bond length is approximately equal to 118.7 pm in the thiocyanate ion.
A small electronegativity difference of 0.03 units exists between the bonded sulfur (E.N = 2.58) and carbon (E.N = 2.55) atoms in the [SCN]– molecular ion. So the S=C bond is only weakly polar. However, a comparatively higher electronegativity difference of 0.49 units is present between the bonded carbon and nitrogen (E. N = 3.04) atoms therefore the C=N bond is also polar in [SCN]–.
The highly electronegative nitrogen atom not only attracts the shared C=N electron cloud but also attracts the electrons shared between S and C atoms.
As a result, the overall polarity effect is enhanced and the electron cloud stays non-uniformly distributed. Partial positively (δ+) charged and partial negatively (δ-) charged poles develop in the molecular ion. Thus thiocyanate [SCN]– is overall polar with a net dipole moment (symbol µ) greater than zero.
The Lewis dot structure of SCN– consists of three different elemental atoms.
There is a carbon (C) atom, a nitrogen (N) atom, and a sulfur (S) atom.
The central C atom is bonded to nitrogen and sulfur via single covalent bonds.
There are a total of 16 valence electrons i.e., 8 electron pairs.
Out of the 8 electron pairs, there are 4 bond pairs and 4 lone pairs.
2 lone pairs of electrons are present on each of the N and O atoms while there is no lone pair on the central C-atom.
What is the molecular geometry and shape of SCN–?
The thiocyanate [SCN]– ion has a linear molecular geometry and shape, identical to its ideal electron pair geometry.
How is the Lewis structure and shape of SCN– similar or different to OCN–?
The thiocyanate [SCN]– ion has a similar shape and geometry to the [OCN]– ion i.e., linear.
However, the best possible Lewis structure of OCN– consists of a single covalent bond on one side of the central C-atom and a triple covalent bond on the other side. A -1 formal charge is present on the oxygen atom.
In SCN– there are double covalent bonds on each side of the central C-atom while a -1 formal charge is present on the nitrogen atom.
Having said that, both SCN– and OCN– contain a total of 16 valence electrons and are monovalent anions.
The actual Lewis structure of each of the thiocyanate and cyanate ions is a hybrid of three resonance structures.
Both S=C and C=N double bonds in SCN– are polar as a certain electronegativity difference exists between the bonded atoms. The molecule possesses a linear shape but the dipole moments of S=C and C=N bonds do not get canceled equally so net µ > 0. Therefore, thiocyanate is a polar molecular ion.
The total valence electrons available for drawing thiocyanate [SCN]– ion Lewis structure is 16.
The negative 1 charge present on the ion accounts for 1 extra valence electron in its Lewis structure.
The [SCN]– ion has an identical electron and molecular geometry or shape i.e., linear.
The S=C=N atoms lie in a planar arrangement, on a straight line therefore the S=C=N bond angle is 180° in SCN– ion.
The SCN– ion has sp hybridization.
The SCN– ion is overall polar in nature.
-1 formal charge is present on the nitrogen atom while zero formal charges are present on the sulfur and carbon atoms in the best possible Lewis representation of SCN– which accounts for an overall negative one charge on the thiocyanate anion.
Vishal Goyal is the founder of Topblogtenz, a comprehensive resource for students seeking guidance and support in their chemistry studies. He holds a degree in B.Tech (Chemical Engineering) and has four years of experience as a chemistry tutor. The team at Topblogtenz includes experts like experienced researchers, professors, and educators, with the goal of making complex subjects like chemistry accessible and understandable for all. A passion for sharing knowledge and a love for chemistry and science drives the team behind the website. Let's connect through LinkedIn: https://www.linkedin.com/in/vishal-goyal-2926a122b/
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