Lewis Structure and Resonance Structures of SCN- (Thiocyanate ion)
Lewis structure and resonance structures of thiocyanate ion is drawn step by step in this tutorial. Total valence electrons of atoms, negative charge are considered to draw the SCN- lewis structure. Thiocyanate ion includes three kinds atoms.
written by: Heshan Nipuna, last update: 29/05/2020
Lewis Structure of thiocyanate ion
Now, we are going to learn, how to draw this lewis structure of ClO3- ion step by step.
Steps of drawing SCN- lewis structure
Following steps are required to draw SCN- lewis structure and they are explained in detail in this tutorial.
- Find total number of electrons of the valance shells of sulfur, nitrogen and carbon atoms and including charge of the -1 charge
- Total electrons pairs in valence shells
- Determine center atom from carbon, nitrogen and sulfur atoms
- Put lone pairs on atoms and mark charges on atoms
- Stability of lewis structure - Check the stability and minimize charges on atoms by converting lone pairs to bonds to obtain the best lewis structure.
Drawing correct lewis structure is important to draw resonance structures of SCN- ion.
Total number of electrons of the valance shells of carbon, nitrogen and sulfur atoms and charge of the anion
- There are three types of atoms; carbon, nitrogen and sulfur. These atoms are located at different groups.
- Carbon is located at group 4 and has four electrons in its valence shell. Nitrogen is at group 5 and contains five electrons in its valence shell. Other atom, sulfur is a group 16 element and has six electrons in its valence shell.
- Total valence electrons given by carbon atom = 4*1 = 4
- Total valence electrons given by nitrogen atom = 5*1 = 5
- Total valence electrons given by sulfur atom = 6*1 = 6
Due to -1 charge, another electrons is added to the total valence electrons count.
- Due to -1 charge, received electrons to valence electrons= 1
- Total valence electrons = 4 + 5 + 6 + 1 = 16
Total valence electrons pairs
Total valance electrons pairs = σ bonds + π bonds + lone pairs at valence shells
Total electron pairs are determined by dividing the number total valence electrons by two. For, SCN- ion, there are 16 valence electrons, so total pairs of electrons are 8.
Center atom of SCN- ion
For being the center atom, ability of having greater valance is a reason. If we consider each element, maximum valence of carbon, nitrogen and sulfur are 4, 5 and 6 respectively. You may know, in so many compounds which are include carbon has four bonds around carbon atom. However, for SCN- ion, sketch is as below.
Sketch of SCN- ion
Lone pairs on atoms
- There are only two (2) bonds around center atom in the sketch of thiocyanate ion; carbon (C-O and C-S). Therefore, now six (8-2) electrons pairs are remaining to mark lone pairs.
- As the next step, mark those six valence electrons pairs on outside atoms (nitrogen and sulfur atoms) as lone pairs. Each outside atom will take three lone pairs. Therefore, total of six electrons pairs are marked on nitrogen and sulfur atoms. Now, there is more electrons pairs to mark on carbon atom.
Check the stability of drawn structure of SCN- ion and reduce charges on atoms by converting lone pairs to bonds
Check charges on atoms and mark them as below. Lowering the charges on atoms is important to obtain the best lewis structure of the ion.
Mark charges on atoms
There are charges on every atoms of above structure. Therfore, drawn structure for SCN- is not a stable structure. Also, when charge of an atom is great (like +2, +3), that structure become more unstable. When a molecule or ion has so many charges on atoms and charge is great that structure is not stable.
Now, we should try to minimize charges by converting lone pair or pairs of outside atoms to bonds. So we convert one lone pair of nitrogen atom as a C-N bond as in the following figure.
Now there is a double bond between carbon atom and nitrogen atom. And charge of carbon atom is reduced from +2 to +1 and for nitrogen, charge is reduced to -1. Still sulfur has a -1 charge.
But, there are still charges on all atoms. If possible, we should reduce charges furthermore. Therefore, a lone pair of sulfur atom is transferred as a S-O bond (See the figure).
Now, there are no charges around carbon atom and sulfur atom. Only nitrogen has a -1 charge. That is an acceptable point because nitrogen is the most electronegative element from these three elements. Therefore, this new structure is the best lewis structure of thiocyanate ion.
Questions
What are the deviations of SCN- lewis structure from usual steps of drawing a lewis structure?
In SCN- ion, carbon atom is the center atom. Usually we think, center atom should be an element which has a higher valence. In SCN-, nitrogen has highest valeence. But, nitrogen is not the center atom of SCN- ion. That is a deviation from usual practise of drawing a lewis structure.
