First IE of `5d` seties element are higher than those of `3d` and `4d` series elements. This is due to:

Options

A. bigger size of atoms of `5d` -series elements than `3d` series elements
B. Greater effective nuclear charge is experienced by valence electrons because of the weak shielding of the nuclear by `4f`-electrons in `5d` series.
C. (A) and (B) both
D. None of these

Explanation

To answer the question regarding why the first ionization enthalpy of 5d series elements is higher than that of 3d and 4d series elements, we can break down the explanation into several steps:

### Step-by-Step Solution:

1. **Understanding Ionization Energy**:
- Ionization energy (IE) is the energy required to remove an electron from an atom in its gaseous state. Higher ionization energy indicates that it is more difficult to remove an electron.

**Hint**: Remember that ionization energy is influenced by the effective nuclear charge and the shielding effect of inner electrons.

2. **Comparing the Series**:
- The 3d, 4d, and 5d series refer to the filling of d orbitals in transition metals. The 3d series includes elements like Scandium (Sc) to Zinc (Zn), the 4d series includes Yttrium (Y) to Cadmium (Cd), and the 5d series includes Hafnium (Hf) to Copernicium (Cn).

**Hint**: Familiarize yourself with the elements in each series and their electron configurations.

3. **Filling of 4f Orbitals**:
- In the case of the 5d series, the 4f orbitals are filled before the 5d orbitals. The 4f orbitals are known to have a poor shielding effect.

**Hint**: Recall that the order of filling orbitals is based on the Aufbau principle.

4. **Understanding Shielding Effect**:
- The shielding effect refers to the reduction in the effective nuclear charge felt by valence electrons due to the presence of inner shell electrons. The 4f electrons do not shield the 5d electrons effectively, leading to a higher effective nuclear charge.

**Hint**: Consider how inner electrons can either shield or not shield the outer electrons from the nucleus.

5. **Effective Nuclear Charge (Z_eff)**:
- Due to the poor shielding effect of the filled 4f orbitals, the effective nuclear charge (Z_eff) experienced by the valence electrons in the 5d series is significantly higher than that in the 3d and 4d series.

**Hint**: Z_eff can be calculated using the formula Z_eff = Z - S, where Z is the atomic number and S is the shielding constant.

6. **Conclusion**:
- Because the valence electrons in the 5d series experience a higher effective nuclear charge, they are held more tightly by the nucleus. This makes it more difficult to remove an electron, resulting in higher first ionization enthalpy compared to the 3d and 4d series.

**Hint**: Summarize the relationship between effective nuclear charge, shielding effect, and ionization energy to reinforce your understanding.

### Final Answer:
The first ionization enthalpy of 5d series elements is higher than that of 3d and 4d series elements due to the poor shielding effect of the filled 4f orbitals, which leads to an enhanced effective nuclear charge on the valence electrons in the 5d series, making them more tightly bound to the nucleus and requiring more energy to remove.