Introduction:
Pentahalides and trihalides are types of chemical compounds that contain halogen atoms bonded to a central atom. The difference between them lies in the number of halogen atoms bonded to the central atom. Pentahalides have five halogen atoms bonded to the central atom, while trihalides have only three. In general, pentahalides are more covalent than trihalides. This can be explained by considering several factors.

Electron Density:
The number of halogen atoms in a pentahalide is greater than in a trihalide. As a result, the electron density around the central atom is higher in pentahalides. This increased electron density leads to stronger covalent bonding between the central atom and the halogen atoms.

Size of the Central Atom:
The size of the central atom also plays a role in determining the covalent nature of the compound. In pentahalides, the central atom is typically larger than in trihalides. The larger size of the central atom allows for more effective overlapping of atomic orbitals, resulting in stronger covalent bonds.

Electronegativity Difference:
The electronegativity difference between the central atom and the halogen atoms is another important factor. In pentahalides, the electronegativity difference is typically smaller than in trihalides. This means that the sharing of electrons between the central atom and the halogen atoms is more equal, resulting in a more covalent bond.

Hybridization:
The hybridization of the central atom also influences the covalent nature of the compound. In pentahalides, the central atom is often sp3d hybridized, which allows for greater orbital overlap and stronger covalent bonding. In contrast, trihalides may have a different hybridization state, such as sp2, which results in weaker covalent bonding.

Conclusion:
In summary, pentahalides are more covalent than trihalides due to factors such as higher electron density, larger size of the central atom, smaller electronegativity difference, and appropriate hybridization. These factors contribute to stronger covalent bonding between the central atom and the halogen atoms in pentahalides.