Question

Identify the elements of alkali metals and alkaline earth metals respectively having highest 1st ionization enthalpy.

• A. \(\text{K}\) and \(\text{Ca}\)
• B. \(\text{Li}\) and \(\text{Be}\)
• C. \(\text{Rb}\) and \(\text{Ba}\)
• D. \(\text{K}\) and \(\text{Sr}\)

Hint:

Ionization enthalpy falls down a group, so the top element of each group is highest.

Answer 1

The Correct Option is B

Step 1: Trend:
Effective nuclear charge rises only slightly down a group, while the distance of the outer electron from the nucleus rises a lot. Removal gets easier.

Step 2: Pick the top members:
The smallest atom in group 1 is Li, and in group 2 it is Be. These have the highest values. Each wrong option contains at least one heavier element (K, Ca, Rb, Ba or Sr), which is easier to ionize.

Final Answer:
The pair is Li and Be, option (B). \[ \boxed{\text{Li and Be}} \]