StemCET Logo

Chemistry Question 57 – JEE-MAIN 2025

← Back
The elements of Group 13 with highest and lowest first ionisation enthalpies are respectively:

Ionisation enthalpy generally decreases down a group due to increasing atomic size and shielding effect.

🥷
Ninja Strategy•Trend Analysis and Anomalies

Recognize that Boron (B) will always have the highest ionization enthalpy in Group 13, and then recall the anomalous trend (due to d and f orbital effects) to identify Indium (In) as having the lowest.

Step 1: Understand the General Trend✦ Active

The first ionization enthalpy generally decreases down a group due to an increase in atomic size and shielding effect, which reduces the effective nuclear charge experienced by the outermost electron. Therefore, Boron (B), being the first element in Group 13, is expected to have the highest ionization enthalpy.

Step 2: Analyze the Anomalous Trend in Group 13○ Expand

Group 13 elements (B, Al, Ga, In, Tl) show an irregular trend in ionization enthalpies. The decrease from Al to Ga is less than expected (or even a slight increase) due to the poor shielding of 3d electrons in Ga. Similarly, Thallium (Tl) has a higher ionization enthalpy than Indium (In) due to the poor shielding of 4f and 5d electrons. The order of first ionization enthalpies is B > Tl > Ga > In > Al.

💡 Teacher's Secret Hint

Remember that the general trend of decreasing ionization enthalpy down a group is often disrupted by the presence of d and f electrons in heavier elements, leading to unexpected increases.

Step 3: Identify Highest and Lowest Values○ Expand

Comparing the first ionization enthalpies (in kJ/mol):

Boron (B):801Aluminium (Al):577Gallium (Ga):579Indium (In):558Thallium (Tl):589

From these values, the highest first ionization enthalpy is for Boron (B) and the lowest is for Indium (In). Thus, the elements are B and In, respectively.

✦ STEM Console utilizes AI models to generate step-by-step explanations and math clues. AI can make mistakes.