Question

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.

• A. lowest and lowest
• B. highest and lowest
• C. lowest and highest
• D. highest and highest

Answer 1

The Correct Option is C

Concept: Periodic trends across a period:

• First ionisation enthalpy increases from left to right.
• Electron gain enthalpy becomes more negative from left to right.

Step 1: {First ionisation enthalpy} Elements at extreme left (alkali metals) have the lowest ionisation enthalpy. Step 2: {Electron gain enthalpy} Elements at extreme right (halogens, excluding noble gases) have the most negative electron gain enthalpy. Thus it is the highest in magnitude. Therefore: \[ \text{lowest and highest} \]