Diphenylmethane consists of two phenyl rings attached to a central methylene ($-\text{CH}_2-$) group. The molecule is symmetrical, meaning both phenyl rings are equivalent. There are four distinct types of hydrogen atoms in the molecule which can be replaced by a chlorine atom to give different structural isomers:

1. Hydrogen atom(s) on the central methylene carbon.
2. Hydrogen atoms at the ortho-positions of the phenyl rings.
3. Hydrogen atoms at the meta-positions of the phenyl rings.
4. Hydrogen atoms at the para-positions of the phenyl rings.

Since there are 4 sets of equivalent hydrogens, replacing one hydrogen with a chlorine atom yields exactly four different monochloro structural isomers .