Formal Charge of Cl in HClO4
Cl in HClO4 has a formal charge of +7.

Explanation:
- The formal charge of an atom in a molecule is calculated by assigning electrons in the molecule to the atoms according to certain rules.
- In HClO4, the Cl atom is bonded to one hydrogen atom and four oxygen atoms.
- Chlorine typically has 7 valence electrons. In HClO4, Cl is surrounded by 1 hydrogen atom and 4 oxygen atoms, making a total of 5 bonds.
- Each bond contributes 2 electrons to the atom, so Cl in HClO4 has 10 valence electrons associated with it.
- The formal charge of an atom is calculated by subtracting the number of lone pair electrons and half the number of bonding electrons from the total number of valence electrons of the free atom.
- In the case of Cl in HClO4, it has 10 valence electrons (7 from Cl, 1 from H, and 2 each from 4 O atoms).
- Cl has 6 lone pair electrons and 4 bonding electrons, resulting in a formal charge of +7 (10 - 6 - 4 = 0).
Therefore, the formal charge of Cl in HClO4 is +7.