Understanding Zeff
The effective nuclear charge (Zeff) is the net positive charge experienced by electrons in an atom. It accounts for the shielding effect of inner electrons, which reduces the full nuclear charge felt by valence electrons.

Factors Affecting Zeff
1. **Nuclear Charge (Z)**: The total number of protons in the nucleus.
2. **Shielding Effect**: Inner electrons shield valence electrons from the full effect of the nuclear charge.

Arrangement of Zn²+, Cd²+, and Hg²+
To compare Zn²+, Cd²+, and Hg²+, we need to assess their respective effective nuclear charges.
- **Zinc (Zn²+)**:
- Atomic Number (Z): 30
- Electron Configuration: [Ar] 3d¹⁰
- Shielding Electrons: 28 (from Ar)
- Zeff = Z - Shielding = 30 - 28 = 2
- **Cadmium (Cd²+)**:
- Atomic Number (Z): 48
- Electron Configuration: [Kr] 4d¹⁰
- Shielding Electrons: 46 (from Kr)
- Zeff = Z - Shielding = 48 - 46 = 2
- **Mercury (Hg²+)**:
- Atomic Number (Z): 80
- Electron Configuration: [Xe] 4f¹⁴ 5d¹⁰
- Shielding Electrons: 78 (from Xe)
- Zeff = Z - Shielding = 80 - 78 = 2

Final Arrangement
Despite being different elements, the Zeff values for Zn²+, Cd²+, and Hg²+ are the same:
- Zn²+ = 2
- Cd²+ = 2
- Hg²+ = 2
Thus, they can be arranged in any order as they exhibit similar effective nuclear charge values.

Conclusion
Understanding Zeff helps in predicting the chemical behavior of ions, emphasizing the importance of effective nuclear charge in atomic structure studies.