Periodic Trends – IIT‑JEE Chemistry Notes
(Focus: Trends that help solve JEE problems quickly and accurately.)
1. Simple Explanation
The periodic table is arranged so that atoms with similar properties recur every 8th column (group). As you move
-
Across a period (left → right):
- Nuclear charge increases → electron cloud is pulled tighter → atomic size shrinks.
- Electrons added to the same shell → shielding does not increase significantly.
-
Down a group (top → bottom):
- A new shell is added → size increases.
- Shielding increases → effective nuclear charge (Z_eff) is roughly constant → properties change more gradually.
These structural changes give rise to systematic periodic trends that are the backbone of chemical reasoning in the JEE.
2. Key Periodic Trends
| Trend | Direction | Reason | Typical JEE Implication |
|---|---|---|---|
| Atomic (covalent) radius | ↑ (down a group) | Extra shell dominates | Reactivity of alkali metals ↑ |
| ↓ (across a period) | ↑ Z_eff pulls electrons closer | Size of halogens ↓ | |
| Effective Nuclear Charge (Z_eff) | ↑ (across a period) | More protons, same shielding | Ionization energy ↑ |
| ≈ constant (down a group) | New shell adds shielding | Trend weaker | |
| First Ionization Energy (IE₁) | ↑ (across a period) | Tighter binding of valence electrons | Noble gases highest IE |
| ↓ (down a group) | Larger radius, more shielding | Alkali metals low IE | |
| Electron Affinity (EA) | ↑ (across a period) | Electrons added to smaller orbitals | Halogens most negative EA |
| ↓ (down a group) | Shielding reduces attraction | Alkali metals EA ≈ 0 | |
| Electronegativity (Pauling) | ↑ (across a period) | ↑ Z_eff, ↑ IE | Nitrides/halides highly electronegative |
| ↓ (down a group) | Larger radius | Alkali metals least | |
| Metallic Character | ↓ (across a period) | ↑ Z_eff, ↑ IE, ↓ EA | Transition metals → non‑metals |
| ↑ (down a group) | Larger radius, low IE | Alkali/alkaline earth metals | |
| Van der Waals Radius | ↑ (down a group) | Outer shell expands | Noble gas sizes ↑ |
| ↓ (across a period) | Inner shells dominate | Size of halogens ↓ | |
| Melting & Boiling Points | ↓ (down a group) for metals | Weaker metallic bonds | Al → Cu → Au |
| ↑ (across a period) for non‑metals | Stronger covalent network | C (diamond) → Si → Ge |
Tip: When a JEE question asks “Which element is more reactive?” or “Which ion will form a stronger bond?”, refer to the trend that directly relates to the property in question.
3. Exceptions & Nuances
| Exception | Why? | JEE Note |
|---|---|---|
| b‑blocks (transition metals) | d‑orbitals shield poorly; trends are less regular | Use d electron count for magnetic and spectroscopic questions |
| p‑blocks with filled d‑shells (e.g., Cu, Zn, Ag, Au) | d‑orbitals are filled → less effective shielding | Lower IE than expected |
| Group 13–15 elements | p‑electron shielding changes irregularly | Compare stability of oxidation states |
| Halogens | EA becomes less negative down the group due to large radius | Hard to predict EA of I vs. Cl |
4. Worked Example
Question (JEE‑style)
Predict the most reactive element among the following: (a) Na, (b) Mg, (c) K, (d) Ca.
Solution
- Identify the group. All are Group 1 or 2 (alkali/alkaline earth).
- Apply trend – Reactivity ↑ ↓ down a group.
- Na (Na⁺) → 3rd period, Mg (2nd period) → 2nd period, K (4th period) → 4th period, Ca (3rd period) → 3rd period.
- Rank by period:
- K (4th) > Na (3rd) ≈ Ca (3rd) > Mg (2nd).
- Check metallic character trend:
- For Group 2, Ca is slightly more reactive than Mg, but still less than Na/K.
- Answer: K is the most reactive.
Why?
- K has the lowest IE (≈ 4.34 eV) and the largest radius (1.38 Å), giving it the highest tendency to lose an electron and form K⁺.
5. Quick Revision Summary
- Atomic radius: ↓ across a period, ↑ down a group.
- Ionization Energy: ↑ across a period, ↓ down a group.
- Electron Affinity: ↑ across a period, ↓ down a group.
- Electronegativity: ↑ across a period, ↓ down a group.
- Metallic Character: ↓ across a period, ↑ down a group.
- Van der Waals radius: Same trend as atomic radius.
- Boiling/Melting points:
- Metals: ↓ down a group, ↑ across a period