Question

Pyrosilicate ion is:

• A. $S{i}_2{O}_4^{4-}$
• B. $S{i}_2{O}_7^{6-}$
• C. $S{i}_2{O}_2^{6-}$
• D. $S{i}_2{O}_2^{7-}$

Answer 1

The correct option is B $S{i}_2{O}_7^{6-}$

In these silicates, two tetrahedral units are joined by sharing oxygen at one corner thereby giving $[S{i}_2{O}_7{]}^{6–}$ units.
Hence correct option is b.

THEORY
Silicates: Silicates are the minerals containing silicon and oxygen atom in tetrahedral $Si{O}_4^{4-}$ units, which are linked together in several patterns. Depending on the way the tetrahedral units are linked, the silicates are classified into the following types.
Ortho-silicates
Pyro-silicates
Cyclic Silicates
Chain Silicates
2-D Sheet Silicates
3-D Sheet Silicates

1. Ortho-Silicates:
These contain discrete $[Si{O}_4{]}^{4–}$ units i.e., there is no sharing of corners with one another.
2. Pyro-Silicates:

• Two tetrahedral units joined by sharing oxygen at one corner. General formula is $[S{i}_2{O}_7{]}^{6–}$

• -ve charge on the O atoms bonded to only 1 Si atom.
• Cyclic-Silicates:
• 2 oxygen atoms/tetrahedron shared to form closed rings.
• General formula $(Si{O}_3^{2–}{)}_n$ or $Si{O}_3{)}_n^{2n–}$

3. Cyclic Silicates:

• 2 oxygen atoms/tetrahedron shared to form closed rings.
• General formula $(Si{O}_3^{2–}{)}_n$ or $(Si{O}_3{)}_n^{2n–}$