In pentagonal bipyramidal structure, the coordination number(a). 7(b). 6(c). 8(d). 10

To determine the coordination number in a pentagonal bipyramidal structure, let’s first understand what a pentagonal bipyramid is.

### Pentagonal Bipyramidal Structure

- A pentagonal bipyramid consists of two pyramids with a pentagonal base that are joined at their bases.
- It has **7 vertices**: 5 vertices at the base of the pentagon and 2 vertices at the apex of each pyramid.

### Coordination Number

The **coordination number** refers to the number of atoms or groups of atoms that are directly bonded to a central atom. In the case of the pentagonal bipyramid, the coordination number is determined by the number of atoms bonded to the central atom.

#### Analysis of Coordination Number:

1. **Base of the Structure**: The 5 atoms at the base of the pentagonal bipyramid are bonded to the central atom.
2. **Apex Atoms**: There are 2 apex atoms, one above and one below the central atom, which are also bonded to it.

Adding these together:

- Number of base atoms = 5
- Number of apex atoms = 2

### Total Coordination Number Calculation:

\[ \text{Coordination Number} = \text{Number of base atoms} + \text{Number of apex atoms} = 5 + 2 = 7 \]

### Conclusion

Therefore, the coordination number of a pentagonal bipyramidal structure is **7**.

Thus, the correct answer is **(a) 7**.