SCN– is the chemical formula for the thiocyanate ion, aka rhodanide. It is a triatomic anion and a conjugate of thiocyanic acid. It is a colorless reactant and has a molar mass of 58.08 g/mol.
Are you aware of thiocyanate’s property of being anti-thyroid? which means it can block active ingestion of inorganic iodide by the thyroid gland.
This article will discuss another very important property of thiocyanate (rhodanide) SCN–, i.e., its polarity.
So, is SCN– polar or non-polar? Continue reading to find out.
Is SCN– polar or non-polar?
Thiocyanate (SCN–) is a polar molecular ion. Itconsists of one sulfur (S) atom, one carbon (C) atom, and one nitrogen (N) atom. The carbon atom is present at the center of the molecular ion, while sulfur and nitrogen atoms occupy terminal positions, one on each side, making a linear molecular shape.
An electronegativity difference of 0.03 units exists between a carbon and a sulfur atom in a C=S bond that is individually very weakly polar. Contrarily, an electronegativity difference of 0.49 units exists between the bonded carbon and nitrogen atoms in the C=N bond in SCN– ion.
Hence, C=N is comparatively a strongly polar covalent bond and possesses a higher specific dipole moment value (symbol µ).
However, it is due to an unequal electronegativity difference that the dipole moment of C=S and C=N bonds do not cancel out each other in the SCN– ion.
The charged electron cloud stays non-uniformly distributed in the molecular ion overall. Thus, SCN– is a polar ion (net µ = 2.64 Debye).
A molecule is polar if there is a non-uniform charge distribution present in it.
If the charge distribution gets equally balanced in different parts, then that molecule or molecular ion is considered non-polar.
The following three factors mainly influence the polarity of a molecule:
The electronegativity difference between two or more covalently bonded atoms
Dipole moment
Molecular geometry or shape
Now, let us discuss the effect of the above three factors one by one to prove that thiocyanate (SCN–) is overall a polar molecular ion.
Factors affecting the polarity of SCN–
Electronegativity
It is defined as the ability of an atom to attract a shared pair of electrons from a covalent chemical bond.
Electronegativity decreases down the group in the Periodic Table of elements while it increases across a period.
Greater the electronegativity difference between bonded atoms in a molecule or molecular ion, the higher the bond polarity.
Sulfur (S) belongs to Group VI A (or 16) of the Periodic Table. The electronic configuration of a sulfur atom is 1s2 2s2 2p6 3s2 3p4. Hence, the S-atom in SCN– has a deficiency of 2 more valence electrons to obtain a complete octet electronic configuration.
Carbon (C) is present in Group IV A (or 14) of the Periodic Table. The electronic configuration of carbon is 1s2 2s2 2p2. As per this electronic configuration, a C-atom has a total of 4 valence electrons. It is thus short of 4 more electrons that are required so that the carbon atom can achieve a complete octet electronic configuration.
Conversely, nitrogen (N) is present in Group V A (or 15) of the Periodic Table. The electronic configuration of nitrogen is 1s2 2s2 2p3. As per this electronic configuration, an N-atom has a total of 5 valence electrons. It is thus short of 3 more electrons that are required so that a nitrogen atom can achieve a complete octet electronic configuration.
Hence, in SCN– ion, the central carbon atom is double covalently bonded to each of the two terminal atoms, including the S-atom and the N-atom.
All 4 valence electrons of the central C-atom get consumed in covalent bonding; thus, there is no lone pair of electrons present on it in the SCN– Lewis dot structure.
However, both terminal atoms carry 2 lone pairs of electrons each. In this way, all the atoms in the thiocyanate ion attain a completely stable octet electronic configuration in their respective shells, via lone pair of electrons and chemical bonding.
Due to the absence of any lone pair of electrons on the central carbon (C) atom, there is no distortion present in the shape and geometry of the molecular ion. It thus attains a linear molecular shape and geometry.
Atom
Electronic configuration
Valence electrons
Carbon (6C)
1s22s22p2
4
Nitrogen (7N)
1s22s22p3
5
Sulfur (16S)
1s22s22p63s23p4
6
The electronegativity of carbon is 2.55, sulfur is 2.58, and that of nitrogen is 3.04.
The electronegativity difference between the bonded carbon and sulfur atoms in the C=S bond is 0.03 units. This is a very small electronegativity difference as per Pauling’s electronegativity scale, so it is considered almost non-polar. However, we know that no bond is purely non-polar unless it is present between two identical atoms.
Thus the C=S bond is considered weakly polar. Therefore, the C-atom gains a partial positive charge (Cδ+) while the S-atom obtains a slight negative charge (Sδ-) in the SCN– ion.
On the other hand, the bonded carbon and nitrogen atoms in the C=N bond possess an electronegativity difference of 0.49 units.
Thus the nitrogen atom present at the side of the SCN– ion gains a partial negative (Nδ-) charge, while the central carbon atom maintains its partial positive (Cδ+) charge.
Dipole Moment
Dipole moment (μ) is a vector quantity that points from the positive pole to the negative pole of a bond or a molecule.
It is mathematically calculated as a product of the magnitude of charge (Q) and charges separation (r). The dipole moment is expressed in a unit called Debye (D).
The dipole moment of a polar covalent bond conventionally points from the positive center to the center of the negative charge.
So, in SCN–, the individual dipole moments of C=S and C=N bonds point from Cδ+ to Sδ- and from Cδ+ to Nδ-, respectively (as shown below).
The highly electronegative nitrogen atom not only attracts the shared C=N electron cloud but also attracts the electrons shared between the S and C atoms.
Molecular geometry
According to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding, SCN– is an AX2E0 or simply AX2-type molecular ion. To one C-atom (A) at the center, two (one C=S, one C=N) bond pairs (X) are attached, and there is no lone pair of electrons (E) present on the central C-atom.
So, the molecular geometry or shape of SCN– is linear which is identical to its ideal electron pair geometry.
Due to the absence of any lone pair of electrons on the central C-atom, there are no lone pair-lone pair or lone pair-bond pair electronic repulsions present to distort the shape and geometry of the molecular ion. The S=C=N bonded atoms thus form a mutual bond angle of 180°.
Although SCN– is linear in shape, the highly electronegative nitrogen atom not only attracts electrons of the C=N bond but also attracts the shared electrons of the C=S bond.
Furthermore, the C=S dipole moment is much smaller than the C=N dipole moment due to varying electronegativity differences.
Hence, the electron cloud in the ion is not uniformly distributed as the two individual dipole moments do not get canceled equally in opposite directions in even the linear molecular shape.
Consequently, thiocyanate (SCN–) is a polar molecular ion overall (net µ = 2.64 Debye).
Difference between polar and nonpolar?
Polar molecule
Non-polar molecule
Atoms must have a difference in electronegativity
Atoms may have the same or different electronegativity values
Unequal charge distribution overall
Equal charge distribution overall
Net dipole moment greater than zero
Net dipole moment equals to zero
Examples include water (H2O), ethanol (CH3CH2OH), ammonia (NH3), sulfur dioxide (SO2), bromine trifluoride (BrF3), thiocyanate (SCN–) ion, hydronium (H3O+) ion, etc.
SCN– has a weakly polar C=S bond due to an electronegativity difference of only 0.03 units between the bonded carbon (E.N = 2.55) and sulfur (E.N = 2.58) atoms.
Contrarily, a strongly polar C=N bond with an electronegativity difference of 0.49 units is also present in SCN– between the bonded carbon (E.N = 2.55) and nitrogen (E.N = 3.04) atoms.
A highly electronegative nitrogen atom not only attracts electrons of the C=N bond but also from the C=S bond.
Hence the charged electron cloud is not uniformly distributed in even the linear molecular shape. The C=S and C=N dipole moments do not get canceled equally in opposite directions.
Consequently, thiocyanate (SCN–) is a polar molecular ion overall (net µ = 2.64 Debye).
Which one is more polar among these linear molecular ions? OCN– vs SCN–.
OCN– has a linear molecular geometry (O-C≡N). It has one C≡N and one C-O bond. C ≡N (electronegativity difference = 0.49 units) dipole moment is much smaller than C-O (electronegativity difference = 0.89 units).
Therefore, the individual dipole moments do not get canceled in the molecular ion overall. Consequently, OCN– has a net non-zero dipole moment (net µ = 1.62 D) and is overall a polar molecular ion.
On the other hand, SCN– also has a linear molecular geometry (S=C=N). The C=S bond has an electronegativity difference of 0.03 units, while the C=N bond has an electronegativity difference of 0.49 units between the bonded atoms.
Hence, due to the unequal dipole moment value of weakly polar C=S and strongly polar C=N bond, the dipole moments do not get canceled at all (net µ = 2.44 D). SCN– is thus a polar molecular ion overall.
In comparison, SCN– (net µ = 2.44 D) is more polar than OCN– (net µ = 1.62 D) owing to a higher net dipole moment value.
Why is SCN– polar while CO2 is a non-polar molecule?
However, in SCN–, the C=S and C=N dipole moments do not get canceled due to varying electronegativity differences on both sides. Hence SCN– is overall polar (net µ > 0).
Contrarily, in CO2, there are two identical C=O bonds. The C=O dipole moments are equal, and thus they get canceled uniformly in opposite directions; thus, it is overall a non-polar molecule (net µ = 0).
How many resonance structures are possible for drawing the SCN– ion?
The following three resonance structures are possible for SCN–.
However, structure 1 (that we used in this article) is the most stable as the -1 formal charge is present on the most electronegative N-atom out of all three atoms available.
Structure 3 is the least stable as it possesses higher formal charges on individual atoms.
The lower the formal charges present on the bonded atom, the greater the stability of a Lewis structure.
Is there a formal charge on the bonded atoms in the SCN– molecule?
Formal charge of an atom = [ valence electrons – non-bonding electrons- ½ (bonding electrons)]
Zero formal charges is present on the S-atom and the central C-atom. In comparison, the N-atom carries a -1 formal charge.
Thus, the overall charge present on the thiocyanate SCN– ion is (0) + (0) + (-1) = -1.
Summary
Thiocyanate (SCN–) is a polar molecular ion.
The electronegativity of carbon is 2.55, sulfur is 2.58, and that of nitrogen is 3.04.
It consists of one weakly polar C=S bond due to an electronegativity of 0.03 units between the bonded C-atom and S-atoms, also one polar C=N bond due to an electronegativity difference of 0.49 units between the bonded C-atom and N-atom.
Thiocyanate (SCN–) has a linear molecular and electron pair geometry with a 180° bond angle.
The charged electron cloud does not stay uniformly distributed in the SCN– linear molecular shape.
Due to the unequal dipole moment values of weakly polar C=S and strongly polar C=N bonds, the dipole moment does not get canceled out.
The net dipole moment in thiocyanate (SCN–) is thus µ = 2.44 Debye, so SCN– is overall a polar molecular ion.
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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