Assertion: Ionisation enthalpies of early actinoids are lower than early lanthanoids.

Reason: Electrons entering 5f orbitals in actinoids experience greater shielding from nuclear charge.

Explanation:

The ionization enthalpy is the energy required to remove an electron from an atom or ion in the gaseous state. The ionization enthalpies of the actinoids (elements in the actinide series) are generally lower than those of the lanthanoids (elements in the lanthanide series). This can be attributed to the following reasons:

1. Effective Nuclear Charge: The effective nuclear charge experienced by an electron is the net positive charge felt by the electron after accounting for the shielding effect of other electrons. In the actinoids, the 5f orbitals are located further from the nucleus compared to the 4f orbitals in the lanthanoids. As a result, the 5f electrons in actinoids experience more shielding from the inner electrons and the effective nuclear charge is reduced. This reduces the attraction between the nucleus and the outermost electrons, making it easier to remove an electron and hence lower ionization enthalpies.

2. Penetration Effect: The 5f orbitals have a higher degree of penetration into the inner electron shells compared to the 4f orbitals. This means that the 5f electrons have a higher probability of being located closer to the nucleus, resulting in a stronger attraction between the nucleus and the electrons. However, due to the greater shielding effect, the overall effect of penetration is reduced, resulting in lower ionization enthalpies.

3. Electron Repulsion: The presence of additional electrons in the 5f orbitals in actinoids leads to increased electron-electron repulsion. This repulsion makes it easier to remove an electron from the atom, resulting in lower ionization enthalpies.

Conclusion:

In conclusion, the lower ionization enthalpies of early actinoids compared to early lanthanoids can be attributed to the greater shielding experienced by the 5f electrons in actinoids, as well as the combined effects of effective nuclear charge, penetration, and electron repulsion.