S-S bond is present in which of the ion pairs

The correct answer is S₄O₆²⁻, S₂O₃²⁻.

To determine which of the ion pairs contain a sulfur-sulfur (S-S) bond, let us examine the chemical structures of the given thionic and oxoacids/ions of sulfur step-by-step:

1. Tetrathionate Ion (S₄O₆²⁻):
The tetrathionate ion consists of a chain of four sulfur atoms. The structure can be represented as:
[O₃S-S-S-SO₃]²⁻
In this structure, the two terminal sulfur atoms are bonded to three oxygen atoms each, while the two central sulfur atoms are bonded to each other and to the terminal sulfur atoms. This forms a continuous chain of sulfur atoms linked by single covalent bonds (S-S-S-S). Thus, S-S bonds are present in S₄O₆²⁻.

2. Thiosulfate Ion (S₂O₃²⁻):
The thiosulfate ion is derived from the sulfate ion (SO₄²⁻) by replacing one of the oxygen atoms with a sulfur atom. The central sulfur atom is bonded to three oxygen atoms and one terminal sulfur atom. The structure can be represented as:
[S=SO₃]²⁻
Here, there is a coordinate or double bond between the central sulfur atom and the terminal sulfur atom (S=S or S-S bond). Therefore, a sulfur-sulfur bond is present in S₂O₃²⁻.

3. Pyrosulfate Ion (S₂O₇²⁻):
The pyrosulfate (or disulfate) ion consists of two sulfur atoms sharing a bridging oxygen atom. Its structure is:
[O₃S-O-SO₃]²⁻
Since the two sulfur atoms are linked via an oxygen bridge (S-O-S bond), there is no direct S-S bond in S₂O₇²⁻.

4. Peroxydisulfate Ion (S₂O₈²⁻):
The peroxydisulfate ion contains a peroxide linkage (-O-O-) between two sulfur atoms. Its structure is:
[O₃S-O-O-SO₃]²⁻
The sulfur atoms are linked via a peroxo bridge (S-O-O-S bond), meaning there is no direct S-S bond in S₂O₈²⁻.

Consequently, direct S-S bonds are present in both S₄O₆²⁻ and S₂O₃²⁻, making this pair the correct choice.