The larger the atomic size, smaller is the value of the ionisation enthalpy. Again higher the screening effect, lesser is the value of ionisation potential.

Hence option (b) has lowest ionisation enthalpy

Explanation:

The ionization enthalpy is the energy required to remove an electron from an atom in its gaseous state. It represents the strength of the attraction between the nucleus and the outermost electron.

In this case, we are given four electronic configurations and we need to determine which one has the lowest ionization enthalpy.

Electronic configurations:
a) 1s2 2s2 sp3
b) 1s2 2s2 2p5 3s1
c) 1s2 2s2 2p6
d) 1s2 2s2 2p5

Analysis:
To determine the ionization enthalpy, we need to consider the following factors:
1. Nuclear charge: The greater the nuclear charge, the stronger the attraction between the nucleus and the electrons.
2. Shielding effect: The presence of inner shells of electrons shields the outermost electrons from the full charge of the nucleus.
3. Electron-electron repulsion: Electrons in the same subshell repel each other, which weakens the attraction between the nucleus and the outermost electrons.

Analysis of each configuration:
a) 1s2 2s2 sp3
- This configuration has a total of 7 electrons.
- The outermost electron is in the sp3 hybrid orbital.
- The electron experiences shielding from the 1s and 2s electrons, but it also experiences electron-electron repulsion from the other electrons in the sp3 orbital.
- The ionization enthalpy will be relatively higher due to the greater electron-electron repulsion and less effective shielding.

b) 1s2 2s2 2p5 3s1
- This configuration has a total of 8 electrons.
- The outermost electron is in the 3s orbital.
- The electron experiences shielding from the 1s, 2s, and 2p electrons.
- The ionization enthalpy will be relatively lower compared to option a) because of the greater shielding and lesser electron-electron repulsion.

c) 1s2 2s2 2p6
- This configuration has a total of 8 electrons.
- The outermost electron is in the 2p orbital.
- The electron experiences shielding from the 1s and 2s electrons but does not experience electron-electron repulsion from other electrons in the same subshell.
- The ionization enthalpy will be relatively higher compared to option b) because of the lesser shielding and greater electron-electron repulsion.

d) 1s2 2s2 2p5
- This configuration has a total of 7 electrons.
- The outermost electron is in the 2p orbital.
- The electron experiences shielding from the 1s and 2s electrons but does not experience electron-electron repulsion from other electrons in the same subshell.
- The ionization enthalpy will be relatively higher compared to option b) because of the lesser shielding and greater electron-electron repulsion.

Conclusion:
Among the given options, option b) with the electronic configuration 1s2 2s2 2p5 3s1 has the lowest ionization enthalpy. This is because the outermost electron experiences the greatest