Anomalies & Diagonal Relationship, Inert-Pair Effect Why this chapter matters for NEET Periodic trends tell you the "default" behavior. NEET loves where the default breaks — the first-member anomalies, diagonal similarities, and the inert-pair effect. Master these, and you can predict which oxidation state is stable (Pb2+ vs Pb4+), which pair behaves alike across groups (Li–Mg), and which p-block element can or cannot exceed the octet (PF5 yes; NF5 no). Refresher view: electronegativity increases across a period and decreases down a group; valency patterns emerge from electron sharing and transfer. Quick refresher: Electronegativity and valency Electronegativity is an atom’s pull on shared electron pairs in a bond. Valency is the typical number of bonds an atom forms to reach a stable configuration. We use these to rationalize periodic patterns and then spot the exceptions that NEET tests. Pauling's difference formula Electronegativity difference relates to extra bond energy (Δ) of A–B compared to A–A and B–B (in eV). Definition of Δ used in Pauling's scale (energies in eV). Mulliken's electronegativity Average of ionization enthalpy (IE) and electron affinity (EA) — in eV. Approximate conversion from Mulliken to Pauling scale. Electronegativity distorts shared electrons (e.g., HCl is polar); valency shows typical bond counts (C forms 4, N forms 3, O forms 2). Trend you should recall instantly for NEET. Appears in the NEET-useful valency rule for s- and p-block. Valency for groups 15–17 equals 8 minus group number (for typical covalency). Hannay–Smith relation for approximate % ionic character from Δ . Electronegativity and electron affinity are the same property. Electron affinity is the energy change when a gaseous atom gains an electron. Electronegativity is an atom’s pull on shared electrons within a bond. Valency is the number of bonds formed (positive integer). Oxidation state is the hypothetical ionic charge (can be positive, negative, or zero). Valency equals oxidation state. Water’s strong polarity arises because O is far more electronegative than H. This makes water an excellent solvent for ions and polar solutes (key to transport in blood). Valency explains why Na (valency 1) combines with Cl (valency 1) in a 1:1 ratio to give NaCl. remember Valency periodicity and covalency limits/expansion in the p-block For s- and p-block: Group number often equals typical valency on the left side (Group 1 → 1, Group 2 → 2, Group 13 → 3, Group 14 → 4). For the right side, typical covalency often follows 8 − group number (Group 15 → 3, Group 16 → 2, Group 17 → 1). Key school-level rule: 2nd-period p-block elements (N, O, F) cannot exceed an octet (no available d-orbitals), so no NF5, OF6, or FF7. In the 3rd period and beyond, many elements can exceed the octet and show higher covalency — examples you must know: phosphorus pentafluoride (PF5), sulfur hexafluoride (SF6), and iodine heptafluoride (IF7). Water dissolving NaCl: polarity from high O electronegativity. Mini chart: valency decides combining ratios (Na+ : Cl− = 1:1; Ca2+ : Cl− = 1:2). Anomalous behavior of first-period p/s members: Li, Be, B, C First members are often different from the rest of their groups. Why? Very small size, very high ionization enthalpy, high electronegativity (for B and C), and — for 2nd-period elements — absence of vacant d-orbitals (so no octet expansion). Together, these change bonding type (more covalent), coordination number, and thermal stability of salts. Element Anomalous behavior (with examples) Reason First member First-member anomalies and reasons Lithium (Li, Group 1) Forms nitride Li3N on heating with N2 (others do not). Li2CO3 and LiNO3 decompose on heating to oxides; Li salts often show higher covalent character (e.g., LiCl). Very small Li+ → strong polarising power; high hydration enthalpy; resembles Mg2+ (diagonal relation). Beryllium (Be, Group 2) BeCl2 is covalent and polymeric (chains/dimers); BeO and Be(OH)2 are amphoteric; extensive complex formation (e.g., [BeF4]2−). Small Be2+ with high charge density → covalency (Fajans’ rules); limited coordination due to tiny size. Boron (B, Group 13) Non-metal; forms only covalent compounds; electron-deficient boranes (e.g., B2H6) with 3-center 2-electron bonds; no simple B3+(aq). Small size and high IE; prefers covalent multicenter bonding; cannot expand octet. Carbon (C, Group 14) Maximum covalency 4 (no octet expansion); strong catenation (chains, rings); multiple bonding; many allotropes (diamond, graphite, fullerenes). Strong C–C bonds; high EN and IE; no d-orbitals; effective pπ–pπ bonding. neet-alert High-yield: Only Li forms Li3N; BeCl2 is covalent and polymeric; B2H6 is electron-deficient; C shows maximum covalency 4 and extreme catenation. Diagonal relationship — when neighbors behave alike Definition: The first element of a group often resembles the second element of the next group diagonally (top-left to bottom-right): Li–Mg, Be–Al, B–Si. Reason: similar charge/radius ratio (charge density), comparable electronegativity, and similar polarising power/hydration enthalpy — leading to similar bonding and stability of salts. Periodic table diagram with three diagonal arrows: Li→Mg, Be→Al, B→Si. Color-code the pairs, show small insets listing 2–3 shared properties (e.g., nitrides formation for Li–Mg, amphoterism for Be–Al, hydrolysis of halides for B–Si). Clean 2D vector, neutral palette, red arrows, no in-image text beyond labels. gpt-image-2 Diagonal pairs highlighted: Li↘Mg, Be↘Al, B↘Si — why these behave alike despite different groups. 2026-05-26T17:04:50.201Z Li (Group 1) and Mg (Group 2) Both form nitrides on heating (Li3N, Mg3N2); their carbonates and nitrates decompose on heating to oxides; halides show higher covalent character compared to heavier congeners. Be (Group 2) and Al (Group 13) Amphoteric oxides/hydroxides [BeO, Be(OH)2; Al2O3, Al(OH)3]; covalent polymeric chlorides (BeCl2, AlCl3); complex formation with F− ([BeF4]2−, [AlF6]3−). B (Group 13) and Si (Group 14) Covalent network solids; halides hydrolyze (BCl3, SiCl4) giving HCl; strong tendency for catenation; reactive hydrides (boranes, silanes). Pair Shared properties (examples) Diagonal relationships: key similarities tip Fast prediction: Start at the first row (period 2) and move one step down and one step right to get the diagonal partner. Expect similarities in bond type and salt stability. Inert-pair effect — why lower oxidation states win down the p-block In heavier p-block elements, the outer ns2 pair becomes relatively less available for bonding — the "inert pair". Poor shielding by intervening d/f electrons increases effective nuclear charge on the s-electrons, stabilizing the ns2 pair. Outcome: lower oxidation states (that do not use the ns2 pair) become more stable down the group. Key examples to remember: Tl+ more stable than Tl3+ (Group 13), Pb2+ more stable than Pb4+ (Group 14), Bi3+ more stable than Bi5+ (Group 15). Compounds in higher states tend to be oxidizing and get reduced to the lower state (e.g., PbO2 → Pb2+). Energy-level schematic for Tl, Pb, Bi: show ns and np orbitals with ns lower and labeled 'inert pair'. Arrows emphasize that using np gives Tl+, Pb2+, Bi3+ stability; higher states (Tl3+, Pb4+, Bi5+) shown as oxidizing, tending to reduce. Clean vector, labels outside, red arrows, no extra text. gpt-image-2 Schematic: ns2 pair sits lower in energy for heavy atoms (Tl, Pb, Bi) and stays unbonded — stabilizing +1/+2/+3 states respectively. 2026-05-26T17:04:51.520Z 13 B → Al → Ga → In → Tl Tl+ (>> Tl3+) Tl3+ (oxidizing) Poor d/f shielding raises Zeff on ns2; the s-pair stays inert. 14 C → Si → Ge → Sn → Pb Pb2+ (>> Pb4+) Pb4+ (oxidizing; reduces to Pb2+) ns2 inert down the group; +2 becomes dominant. 15 N → P → As → Sb → Bi Bi3+ (>> Bi5+) Bi5+ (oxidizing; reduces to Bi3+) Stronger inert-pair effect towards Bi stabilizes +3. Group Elements down the group More stable lower state (downwards) Less stable higher state Reason (NEET-level) Inert-pair effect across heavy p-block remember Industrial links: Pb(IV) compounds (like PbO2) are strong oxidants and readily reduce to Pb(II) — consistent with Pb2+ stability; lead–acid batteries exploit Pb/PbO2 ⇄ PbSO4. Low-melting Bi-based alloys (Wood’s metal: Bi–Pb–Sn–Cd, melts around 70°C) are used in safety devices. Thallium(I) salts historically enabled very low-temperature thermometry. Metals, non-metals, and metalloids — the staircase Across the periodic table, a zigzag staircase separates metals (left/bottom) from non-metals (right/top). Elements along/near this line show intermediate (semimetal) behavior — metalloids. Common metalloids: B, Si, Ge, As, Sb, Te, Po (At is often considered borderline in some sources). 2026-05-26T17:04:51.655Z Periodic table with a staircase: metals (blue), non-metals (green), metalloids on the zigzag (orange). gpt-image-2 Full periodic table, color-overlay: metals blue, non-metals green, metalloids (B, Si, Ge, As, Sb, Te, Po) orange along staircase. Annotate the diagonal line, minimal labels, clean 2D vector style, no extra caption text embedded. Categories and examples to memorize Category Examples (typical) Metals Na, Mg, Al, Fe, Cu, Zn, Sn, Pb Non-metals H, C (diamond/graphite), N2, O2, S, P, Cl2, Br2, I2 Metalloids B, Si, Ge, As, Sb, Te, Po First-member anomaly is only because of small size. Small size is a major cause, but high ionization enthalpy, higher electronegativity, and for period-2 elements the absence of vacant d-orbitals (no octet expansion) are equally important. Diagonal relationship is just a coincidence. It arises from similar charge/radius ratio (charge density), comparable electronegativity, and similar polarising power and hydration enthalpy. Higher states do exist (e.g., PbO2, TlCl3, BiF5) but are less stable and often act as oxidizing agents, reverting to the lower, inert-pair-stabilized state. Inert-pair means the element won’t form compounds in higher oxidation states. Diagonal pairs: "Li–Mg, Be–Al, B–Si" → Learn Maths Before Algebra; Basic Sine. Exam-crisp takeaways Second-period p-block (N, O, F) cannot exceed octet; PF5, SF6, IF7 are valid for heavier congeners. Only Li forms stable nitride Li3N on heating; BeCl2 is covalent and polymeric; B2H6 is electron-deficient. Diagonal: Li–Mg, Be–Al, B–Si — learn 2–3 similarities each. Inert-pair: Tl+, Pb2+, Bi3+ more stable down their groups; higher states are oxidizing.