The error in the Lewis dot diagram is:
**The Ca atom should transfer only one electron to Cl, and the second electron should be transferred to another Cl atom.**
This is because calcium (Ca) is in Group 2 and has two valence electrons. When it forms an ionic bond with two chlorine (Cl) atoms, it transfers both of its electrons—one to each Cl atom. Therefore, the correct formula for this compound would be CaCl₂, indicating that one calcium atom bonds with two chlorine atoms. The representation in the diagram suggests that calcium is only interacting with one chlorine atom, which is not correct for the compound CaCl₂.