Home » Chemistry » Polarity » Is SCN- Polar or Nonpolar? – Polarity of Thiocyanate ion

Is SCN- polar or nonpolar? - Polarity of SCN-

is scn- polar or nonpolar

SCNis 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 SCNpolar or non-polar?

Thiocyanate (SCN) is a polar molecular ion. It consists 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, SCNis a polar ion (net µ = 2.64 Debye).

Name of moleculeThiocyanate (SCN)
Bond type

C=S weakly polar covalent

C=N polar covalent

Molecular geometryLinear
Polar or Non-polar?Overall polar
Dipole moment2.64 D
Bond angle180°

What makes a molecule polar or non-polar?

A molecule is polar if there is a non-uniform charge distribution present in it.

polar molecule

If the charge distribution gets equally balanced in different parts, then that molecule or molecular ion is considered non-polar.

Nonpolar molecule

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 SCNhas 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.

what is scn- lewis structure

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 present in scn-

AtomElectronic configurationValence electrons
Carbon (6C)

1s22s22p2

4
Nitrogen (7N)1s22s22p35
Sulfur (16S)1s22s22p63s23p46

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.

polarity of bonds in scn-

scn- dipole charges

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).

what is dipole moment

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).

dipole moment of scn-

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°.

why is scn- polar molecule

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 moleculeNon-polar molecule
Atoms must have a difference in
electronegativity
Atoms may have the same or different electronegativity values
Unequal charge distribution overallEqual charge distribution overall
Net dipole moment greater than zeroNet 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.Examples include oxygen (O2), nitrogen (N2), methane (CH4), ethane (C2H6), propane (C3H8), ethyne (C2H2), silicon dioxide (SiO2), silane (SiH4), silicon tetrachloride (SiCl4), etc.

Also, check –

FAQ

Why SCN is a polar molecular ion?

  • 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.

polarity of scn- vs ocn-

Why is SCN polar while CO2 is a non-polar molecule? 

Both SCN and CO2 possess a linear shape and molecular geometry.

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).

Why is SCN- polar while CO2 is a non-polar molecule

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.

How many resonance structures are possible for drawing the SCN- ion

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)]

For the central C-atom

  • Valence electrons = 4
  • Bonding electrons = 8
  • Non-bonding electrons = 0

∴ Formal charge on central C-atom = 4 – 0 – 8/2 = 4 – 0 – 4 = 4 – 4 = 0

For N-atom

  • Valence electrons = 5
  • Bonding electrons = 4
  • Non-bonding electrons = 4

∴ Formal charge on double bonded N-atom = 5 – 4 – 4/2 = 5 – 4 – 2 = 5 – 6 = -1

For S-atom

  • Valence electrons = 6
  • Bonding electrons = 4
  • Non-bonding electrons = 4

∴ Formal charge on double bonded S-atom = 6 – 4 – 4/2 = 6 – 4 – 2 = 6 – 6 = 0

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.
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Vishal Goyal author of topblogtenz.com

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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