S₂O₃^2- (Thiosulfate ion) Resonance Structures

Thiosulfate ion (S₂O₃^2-) contains two sulfur atoms and three oxygen atoms with -2 charge. Six resonance structures can be drawn for S₂O₃^2- ion and three of them are stable. Lewis structure of S₂O₃^2- is used to relevant resonance structures.

Lewis structure of thiosulfate ion (S₂O₃^2-)

S₂O₃^2- ion lewis structure contains a S=S bond. Center atom is a sulfur atom and all three oxygen atoms are joint with that sulfuratom. Two oxygen atoms have -1 charges and other oxygen atom and sulfur atom do not have charges. Lewis structure of S₂O₃^2- ion is important because it is required to draw resonance structures of thiosulfate ion.

Resonance structures of S₂O₃^2- ion

Let's draw six resonance structures for the S₂O₃^2- ion.

Lone pairs, charges and bonds of S₂O₃^2- ion

• When we draw resonance structures, we convert lone pairs to bonds and bonds to lone pairs when it is possible. When we do this, you should be careful to protect stability of ion, octal rule and etc.
• In lewis structure of S₂O₃^2- ion, there are three lone pairs (in the last shell) in two oxygen atoms and each of them have -1 charge.
• There is another oxygen atom making a double bond with sulfur and does not have a charge.
• Other sulfur has made a double bond with center sulfur atom.

Steps to draw resonance structures for S₂O₃^2-

You should follow these guidelines in the drawing resonance structures for any molecule or ion.

1. Structure (positions) of lewis ion should not be changed in resonance structures.
2. To obtain stable resonance structures, negative charges should be held on most electronegative elements.
3. If ion has an overall charge should not be changed though charge of atoms are changed.
4. Following octal rule is a must for first two period atoms like oxygen, nitrogen. But atoms such as phosphorous, sulfur does not follow octal rule.

Step by step procedure

• You can convert a lone pair of one oxygen atom which already has three lone pairs to make a bond with center sulfur atom. With that, total electrons around sulfur atom is going to be fourteen. It is not an issue because phosphorous sulfur keep more than eight electrons in its valence shell (sulfur has 3d orbitals which help to keep more than eight electrons).

• With that electron transferring, number of bonds around the sulfur atom become 6 and sulfur atom gains -1 charge. Therefore, this is not a best resonance structure because oxygen should gain the negative charges (electronegativity of oxygen is higher than sulfur).
  
  
• To obtain a stable resonance structure, convert a bond of former double bond (S=O) to a lone pair as in the above figure.

Now, center sulfur atom does not have a charge and two oxygen atom has charges. As like this, we can convert lone pairs to bonds and vice versa to draw six resonance structures of phosphate ion. Position of negative charges is changed when we drew new resonance structures.

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