Which one amongst the following has highest ionization enthalpy?

oxygen

The correct answer is option 1. Oxygen.

Ionization enthalpy refers to the minimum energy required to remove an electron from a gaseous atom or molecule in its ground state. Generally, factors like atomic size and nuclear charge influence this energy.

Within Group 16 (chalcogens), as you move down the group, atomic size increases. This means the outermost electron experiences a weaker nuclear pull, making it easier to remove and decreasing the ionization enthalpy.

However, the trend in ionization enthalpy within Group 16 is also affected by electronic configuration. Oxygen has the configuration \(1s^22s^22p^4\), with two unpaired electrons in the p orbitals. The repulsion between these two electrons slightly destabilizes the electron cloud, making it slightly easier to remove an electron compared to the other elements in the group.

Therefore, despite the increasing atomic size, oxygen has the highest ionization enthalpy due to the electronic repulsion in its p orbitals. Sulphur \(1s^22s^22p^63s^23p^4\), selenium \(1s^22s^22p^63s^23p^63d^{10}4s^24p^4\), and tellurium \(1s^22s^22p^63s^23p^63d^{10}4s^24p^64d^{10}5s^25p^4\) have more electrons, but these electrons are located in inner orbitals or paired in the outer p orbitals, resulting in a stronger nuclear pull and higher ionization enthalpies compared to oxygen.

So, although atomic size generally trends with ionization enthalpy within a group, electronic configuration can play a significant role in specific exceptions like oxygen in Group 16.