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Chemistry Question 58 – JEE-MAIN 2026

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In a period, the first ionisation enthalpy of the element at extreme left and the negative electron gain enthalpy of the extreme right element, except noble gases, are respectively.

Recall the general trends of ionization enthalpy and electron gain enthalpy across a period in the periodic table.

Step 1: Analyze First Ionization Enthalpy Trend✦ Active

Across a period, the effective nuclear charge increases, and the atomic size decreases. This makes it harder to remove an electron. Therefore, the first ionization enthalpy generally increases from left to right in a period. Elements at the extreme left (alkali metals) have the lowest first ionization enthalpy in their respective periods.

Step 2: Analyze Negative Electron Gain Enthalpy Trend○ Expand

Electron gain enthalpy is the energy change when an electron is added to a neutral gaseous atom. A more negative value indicates a greater tendency to accept an electron. Across a period, the effective nuclear charge increases, and atomic size decreases, leading to a stronger attraction for incoming electrons. Thus, electron gain enthalpy generally becomes more negative from left to right. Halogens (elements at the extreme right, excluding noble gases, which have positive electron gain enthalpies) have the most negative (highest negative) electron gain enthalpies in their respective periods.

Step 3: Combine the Trends○ Expand

Based on the trends: the first ionization enthalpy of the element at the extreme left is 'lowest', and the negative electron gain enthalpy of the extreme right element (except noble gases) is 'highest'. Therefore, the correct combination is 'lowest and highest'.

💡 Teacher's Secret Hint

Remember to exclude noble gases when considering electron gain enthalpy trends, as they have stable electron configurations and typically positive electron gain enthalpies.

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