Periodic Table and Periodicity / Block Elements
Why This Chapter Matters
Periodic table and block elements together account for 8-14 marks in JEE — especially p-block and d-block reactions. Trends, exceptions, and important reactions are tested every year.
Core Concepts
1. Periodic Trends
Atomic Radius:
Period (L→R): decreases (more protons, more nuclear attraction, same shell)
Group (top→bottom): increases (new shells added)
Exceptions: Ga smaller than Al (d-block contraction). Tl larger than expected.
Ionisation Energy (IE):
Period (L→R): generally increases (but exceptions: IE of B < Be, IE of O < N)
Reason for exception: Be has fully-filled 2s², N has half-filled 2p³ (extra stable)
Group (top→bottom): decreases (easier to remove electron from larger atom)
Electron Affinity:
Period: generally increases (but Cl > F due to smaller size of F → more repulsion)
F is most electronegative but Cl has highest EA.
Electronegativity:
F > O > N > Cl > Br > C > H > ... (most to least electronegative)
2. s-Block Elements (Groups 1, 2)
Alkali metals (Li, Na, K, Rb, Cs, Fr):
Soft metals, low mp. React vigorously with water. Strong reducing agents.
2Na + 2H₂O → 2NaOH + H₂↑
Flame colours: Li(red), Na(yellow), K(violet), Rb(red-violet), Cs(blue)
Anomalous behaviour of Li: resembles Mg (diagonal relationship)
Alkaline earth metals (Be, Mg, Ca, Sr, Ba):
Harder than alkali metals. Ca,Sr,Ba react with water. Be doesn't react with water.
Anomalous behaviour of Be: resembles Al (diagonal relationship)
3. p-Block Elements (Groups 13-18)
Group 13 (B, Al, Ga, In, Tl):
Boron: non-metal, forms covalent compounds, boron hydrides (boranes)
Aluminium: amphoteric (reacts with both acid and base)
2Al + 6HCl → 2AlCl₃ + 3H₂ | 2Al + 2NaOH + 2H₂O → 2NaAlO₂ + 3H₂
Group 15 (N, P, As, Sb, Bi):
Nitrogen: N₂ very stable (triple bond). Forms oxides (NO, NO₂, N₂O₃, N₂O₅).
Phosphorus: white P (reactive) vs red P (stable, polymeric)
Nitric acid: 3Cu + 8HNO₃(dil) → 3Cu(NO₃)₂ + 2NO + 4H₂O
Group 16 (O, S, Se, Te, Po):
Oxygen: most electronegative after F. Forms oxides with almost everything.
Sulphur: forms SO₂ and SO₃. H₂SO₄ (conc) is oxidising and dehydrating agent.
Group 17 — Halogens (F, Cl, Br, I):
Oxidising agents (F strongest). React with metals, H₂, water.
F₂ + H₂O → HF + HOF (no HClO₄ analogue for F)
Cl₂ + NaOH (cold, dil) → NaCl + NaOCl + H₂O (bleaching powder at industrial scale with Ca(OH)₂)
Cl₂ + NaOH (hot, conc) → NaCl + NaClO₃ + H₂O
4. d-Block (Transition Elements)
Properties: variable oxidation states, coloured compounds, catalytic activity, complex formation.
Electronic config: (n-1)d^1-10 ns^1-2
Common oxidation states and colours:
Mn²⁺ (pale pink), Mn⁴⁺ (MnO₂ black), Mn⁷⁺ (KMnO₄ purple)
Fe²⁺ (pale green), Fe³⁺ (yellow-brown)
Cu⁺ (white CuCl), Cu²⁺ (blue)
Cr³⁺ (green), Cr⁶⁺ (orange dichromate, yellow chromate)
PYQs
2024: Which has highest IE₁: N, O, F, Ne?
Ne (noble gas, highest IE). Among N,O,F: N > O (half-filled 2p³ extra stable). Order: Ne>F>N>O.
2023: Why does F have lower electron affinity than Cl despite being more electronegative?
F is very small → added electron experiences strong electron-electron repulsion from existing electrons in compact 2p orbital. Cl has larger 3p orbital → less repulsion → higher EA.
2022: Why transition metals show variable oxidation states?
Because both (n-1)d and ns electrons have similar energies. Transition metals can lose varying numbers of electrons from both subshells.