The trend in ionisation enthalpy of transition element is not regular because

Options

A. Removal of one electron alters the relative energies of 4s and 3d orbitals
B. Due to different E.C. (stability)
C. Poor screening of 3p orbital
D. Due to decrease in effective nuclear charge

Answer

To address the question regarding the irregular trend in ionization enthalpy of transition elements, we can break down the explanation into a series of logical steps:

### Step-by-Step Solution:

1. **Understanding Ionization Enthalpy**:
- Ionization enthalpy (or ionization energy) is the energy required to remove an electron from an atom in its gaseous state. For transition elements, this involves the removal of electrons from the d-orbitals.

**Hint**: Remember that ionization energy is related to how tightly electrons are held by the nucleus.

2. **Presence of d-Orbitals**:
- Transition elements have partially filled d-orbitals. The presence of these d-orbitals contributes to the complexity of their electronic structure, leading to variable oxidation states.

**Hint**: Consider how the filling of d-orbitals affects the stability and energy levels of electrons.

3. **Variable Oxidation States**:
- Transition metals can exhibit multiple oxidation states due to the involvement of both the 4s and 3d electrons in bonding. This variability affects the energy required to remove electrons, leading to irregular trends in ionization enthalpy.

**Hint**: Think about how different oxidation states can influence the stability of an atom.

4. **Alteration of Orbital Energies**:
- When an electron is removed, the relative energies of the 4s and 3d orbitals change. This alteration can lead to unexpected changes in the ionization enthalpy values as you move across the series of transition metals.

**Hint**: Reflect on how the removal of an electron can impact the remaining electrons and their energy levels.

5. **Conclusion**:
- The irregular trend in ionization enthalpy of transition elements can be attributed to the combination of variable oxidation states, the presence of filled and partially filled d-orbitals, and the alteration of energy levels of the 4s and 3d orbitals upon electron removal.

**Hint**: Summarize the key factors that contribute to the irregularity in ionization enthalpy trends.

### Final Answer:
The trend in ionization enthalpy of transition elements is not regular because the removal of one electron alters the relative energy of the 4s and 3d orbitals, and these elements exhibit variable oxidation states and have valence d-orbitals.