Number of Sigma Bonds in Cl2O7: 8

Explanation:
In order to determine the number of sigma bonds present in Cl2O7, we need to first understand the structure of the molecule.

1. Molecular Formula:
The molecular formula of Cl2O7 represents a compound composed of two chlorine (Cl) atoms and seven oxygen (O) atoms.

2. Lewis Structure:
To determine the arrangement of atoms and the bonding pattern, we can draw the Lewis structure of Cl2O7.

- Chlorine (Cl) is a Group 7A element and has 7 valence electrons.
- Oxygen (O) is a Group 6A element and has 6 valence electrons.

Since we have two chlorine (Cl) atoms and seven oxygen (O) atoms, the total number of valence electrons is calculated as follows:
2(Cl) + 7(O) = 2(7) + 7(6) = 21 + 42 = 63 valence electrons

To minimize electron repulsion, the molecule adopts an arrangement where each oxygen (O) atom is bonded to the central chlorine (Cl) atom. The remaining valence electrons are distributed around the atoms to satisfy the octet rule.

3. Bonding Pattern:
In Cl2O7, each oxygen (O) atom forms a single bond (sigma bond) with the central chlorine (Cl) atom. This results in seven sigma bonds between the oxygen (O) and chlorine (Cl) atoms.

Additionally, each chlorine (Cl) atom is bonded to the other chlorine (Cl) atom through a single bond, resulting in one more sigma bond.

Therefore, the total number of sigma bonds in Cl2O7 is 7 (O-Cl bonds) + 1 (Cl-Cl bond) = 8 sigma bonds.

Conclusion:
The correct answer is '8'. Cl2O7 has a total of eight sigma bonds, which include seven oxygen-chlorine (O-Cl) sigma bonds and one chlorine-chlorine (Cl-Cl) sigma bond.