**56.** Match **List-I** with **List-II**: **List-I:** - (A) \( \text{Al}^{3+} < \text{Mg}^{2+} < \text{Na}^+ < \text{F}^- \) - (B) \( \text{B} < \text{C} < \text{O} < \text{N} \) - (C) \( \text{B} < \text{Al} < \text{Mg} < \text{K} \) - (D) \( \text{Si} < \text{P} < \text{S} < \text{Cl} \) **List-II:** - (I) Ionisation Enthalpy - (II) Metallic Character - (III) Electronegativity - (IV) Ionic Radii Choose the **correct answer** from the options given below: - (1) A-IV, B-I, C-III, D-II - (2) A-II, B-III, C-IV, D-I - (3) A-IV, B-I, C-II, D-III - (4) A-III, B-IV, C-II, D-I

Key Periodic Trends You Need

1. Ionic Radii in Isoelectronic Series
   For ions with the same number of electrons, radius mainly depends on nuclear charge $Z$. More $Z \Rightarrow$ stronger pull $\Rightarrow$ smaller ion.

2. Ionisation Enthalpy (IE)
   In one period IE usually rises left→right because electrons are pulled more tightly. There are small dips (e.g. $\text{B} < \text{Be}$, $\text{O} < \text{N}$) due to subshell arrangements.

3. Metallic Character
   • Increases down a group (outer electron farther away, easier to lose).
   • Decreases across a period (nuclear pull increases, electrons harder to lose).

4. Electronegativity (EN)
   Generally climbs left→right in a period and falls top→bottom in a group.

How a student should think step-by-step

1. Look for an isoelectronic sequence: (A) has $\text{Al}^{3+}, \text{Mg}^{2+}, \text{Na}^+, \text{F}^-$. All have 10 electrons → property must be ionic radius.
2. Find a list with the special B, C, O, N irregularity: (B) mimics the well-known IE pattern with two dips, signalling ionisation enthalpy.
3. Search for a trend that grows down & left: (C) $\text{B} < \text{Al} < \text{Mg} < \text{K}$ clearly shows increasing metallic character.
4. Anything left is likely EN: (D) climbs neatly across the period (Si → Cl), matching electronegativity.

Thus we connect:

• A → Ionic Radii (IV)
• B → Ionisation Enthalpy (I)
• C → Metallic Character (II)
• D → Electronegativity (III)

What is the question like?

We have four different orders of elements/ions. We must decide which property each order is describing:

1. Sizes of ions
2. How much energy it takes to pull off an electron (ionisation enthalpy)
3. How easily an atom grabs electrons (electronegativity)
4. How “metal-like” the element behaves (metallic character).

How do we decide?

Imagine lining up kids from shortest to tallest, or from calm to hyper. Each property is a kind of "line-up":

• Ionic size – Think of the same number of marbles (electrons) squeezed by magnets (nuclear charge). More magnets (protons) pull marbles tighter → smaller ion.
• Ionisation enthalpy – Pulling a lollipop (electron) from a kid. The stronger the kid holds it, the more energy you need.
• Electronegativity – How greedy a kid is for other kids’ lollipops.
• Metallic character – How likely the kid is to share electrons (metals love to share). It grows as you move down a group and falls as you move left → right in a period.

By matching each list to the correct "line-up", we find the answer.

Step-by-Step Matching

1. Sequence (A): $\text{Al}^{3+} < \text{Mg}^{2+} < \text{Na}^+ < \text{F}^-$ All ions are isoelectronic (10 e⁻). In such a series, $\text{larger } Z \;\Rightarrow\; \text{smaller radius}$ The given order is smallest → largest radius, hence Ionic Radii (IV).

2. Sequence (B): $\text{B} < \text{C} < \text{O} < \text{N}$ Experimental first-IE values (kJ mol⁻¹) roughly: 801 < 1086 < 1314 < 1402. This ascending order (with O lower than the general trend) matches Ionisation Enthalpy (I).

3. Sequence (C): $\text{B} < \text{Al} < \text{Mg} < \text{K}$ Moving down & left, metallic nature strengthens. Order fits Metallic Character (II).

4. Sequence (D): $\text{Si} < \text{P} < \text{S} < \text{Cl}$ Electronegativity (Pauling): 1.8 < 2.1 < 2.5 < 3.0. Therefore Electronegativity (III).

Final Matching

$\boxed{\text{A-IV, B-I, C-II, D-III}}$

Comparing with the options ⇒ Option (3) is correct.