JEE Main Chemistry Question (2026) — Solution

Step-by-step solution

In a period, as we move from left to right, the atomic size decreases and the effective nuclear charge increases. Due to the large atomic size and low effective nuclear charge, the element at the extreme left (alkali metal) has the lowest first ionisation enthalpy in the period. As we move to the extreme right (excluding noble gases), the elements (halogens) have the smallest atomic size and highest effective nuclear charge in the period. They require only one electron to complete their octet. Thus, they have the highest negative electron gain enthalpy. Therefore, the first ionisation enthalpy of the extreme left element is the lowest, and the negative electron gain enthalpy of the extreme right element is the highest. Answer: lowest and highest