What do you mean be denticity and chelation?
The number of atoms through which the ligands can donate a pair of electrons is called denticity of a ligand. Ligands on this basis can be unidentate, tredentate or polydentate.
When coordination of central metal atom or ion with ligand takes place from two or more groups of the same ligands in such a way that five or six membered rings are formed with metal atom or ion then it is called chelation and such ligands is called chelating ligands.
Therefore all didentate or polydentate ligands are chelating ligands.