JEE Main Inorganic Chemistry Exceptions List + 25-Question Quiz

The most important JEE Main inorganic chemistry exceptions are the Cr and Cu electronic configurations, ionisation enthalpy anomalies like Be > B and N > O, chlorine's higher electron gain enthalpy than fluorine, the low F–F bond enthalpy, inert pair effect, and anomalous behaviour of Li, Be and B. Most questions test these straight from NCERT.

This page puts all the key inorganic chemistry exceptions for JEE Main in tables with short reasons, and ends with a 25-question self-quiz and a separate answer key. Plan your revision cycles around the JEE Main 2027 exam date.

Tip: Values below are rounded standard values of the kind used in NCERT. JEE asks for orders and reasons, not exact numbers, so focus on which is higher and why.

Which elements have exceptional electronic configurations?

Chromium ([Ar]3d5 4s1) and copper ([Ar]3d10 4s1) are the two you must know. Their heavier group members Mo, Ag and Au behave the same way. Palladium is special: [Kr]4d10 with no 5s electron. The reason is the extra stability of half-filled and fully filled subshells, due to higher exchange energy and symmetry.

Trap: cations lose 4s electrons before 3d, so Fe2+ is [Ar]3d6 and Cu+ is [Ar]3d10.

What are the ionisation enthalpy exceptions?

First ionisation enthalpy normally rises across a period, but Be > B, N > O, Mg > Al and P > S. The reasons are the extra stability of fully filled s and half-filled p subshells, and the fact that a p electron is easier to remove than an s electron. Down a group, poor shielding by d and f electrons causes more exceptions.

Also readJEE Main 2027 Exam Date, Registration and Last Date: Full NTA Schedule

Why is chlorine's electron gain enthalpy more negative than fluorine's?

Fluorine's 2p subshell is very small, so an extra electron faces strong repulsion from the electrons already there. Chlorine's 3p subshell is larger and less crowded, so it releases more energy when it gains an electron. The same size effect makes sulphur's electron gain enthalpy more negative than oxygen's.

Which size, density and melting point trends break?

The key anomalies are: Ga is smaller than Al, Zr and Hf are almost the same size, K is less dense than Na, Ca is the least dense alkaline earth metal, and Ga melts at about 303 K. Poor shielding by d and f electrons explains the size exceptions.

• Ga < Al in size: about 135 pm vs 143 pm; the ten 3d electrons in Ga shield poorly.
• Zr ≈ Hf: about 160 pm and 159 pm, due to lanthanoid contraction.
• Density: K (0.86 g/cm3) is lighter than Na (0.97). In group 2, Ca (1.55) is lighter than Be (1.84) and Mg (1.74).
• Melting points: Ga is lowest in group 13 (about 303 K); Mg is lowest in group 2; Zn has the lowest enthalpy of atomisation in the 3d series.
• Ionic mobility: Li+ is the most hydrated alkali ion, so its mobility in water is lowest: Li+ < Na+ < K+ < Rb+ < Cs+.

What is the inert pair effect and where does it show up?

The inert pair effect is the reluctance of the ns2 electrons of heavy p-block elements to take part in bonding, due to poor shielding by d and f electrons. So the oxidation state two less than the group valency becomes more stable down the group. It is strongest for Tl, Pb and Bi.

Also readJEE Main Result 2026: How to Check, NTA Score and Percentile vs Rank

What is the anomalous behaviour of Li, Be and B?

The first element of each group behaves differently because of its very small size, high charge density, high electronegativity and lack of d orbitals. Lithium, beryllium and boron show this most clearly, and each resembles the element diagonally below-right of it.

Lithium

• Burns in air to give mainly Li2O; Na gives the peroxide Na2O2; K, Rb and Cs give superoxides such as KO2.
• Reacts directly with N2 to form Li3N; other alkali metals do not.
• Li2CO3 decomposes on heating to Li2O and CO2; LiNO3 gives Li2O, NO2 and O2, while other alkali nitrates give nitrite and O2.
• Strongest reducing agent in water (E° about −3.04 V) because of its very high hydration enthalpy.

Beryllium

• BeO and Be(OH)2 are amphoteric; the rest of group 2 gives basic oxides.
• BeCl2 is covalent: a chain polymer in the solid, a chloro-bridged dimer in vapour, and a linear monomer at about 1200 K.
• Be and Mg give no flame colour; Ca gives brick red, Sr crimson and Ba apple green.

Boron

• Forms no B3+ ion; maximum covalency is 4, so it gives [BF4]− while Al gives [AlF6]3−.
• Lewis acid strength is BF3 < BCl3 < BBr3 < BI3, because of strong pπ–pπ back bonding in BF3.
• Boric acid is a weak monobasic Lewis acid: it accepts OH− from water rather than giving H+.

Which pairs show a diagonal relationship?

Li–Mg, Be–Al and B–Si show a diagonal relationship. Moving right increases charge and moving down increases size, so the diagonal elements end up with similar charge-to-radius ratio (polarising power) and similar chemistry.

What are the key anomalies in hydrides, oxides, halides and oxoacids?

The main anomalies come from hydrogen bonding (HF, H2O, NH3), the absence of d orbitals in second-period elements (N, O, F), and weak single bonds between small atoms. These show up as unusual boiling points, missing compounds and unexpected acid strengths.

What are the key d-block and coordination compound exceptions?

Focus on four ideas: copper's positive reduction potential, Cu+ disproportionation in water, colourless d0 and d10 ions, and how ligand strength decides the geometry and magnetism of Ni, Co and Fe complexes.

• Copper: E°(Cu2+/Cu) is about +0.34 V, the only positive value among 3d M2+/M couples, so Cu does not liberate H2 from dilute HCl.
• Cu+ in water: disproportionates to Cu2+ and Cu, because the much higher hydration enthalpy of Cu2+ outweighs the second ionisation enthalpy.
• Colourless ions: Sc3+, Ti4+ (d0) and Cu+, Zn2+ (d10) have no d–d transition. Anhydrous CuSO4 is white; CuSO4·5H2O is blue.
• Colour without d electrons: KMnO4 (purple) and K2Cr2O7 (orange) are coloured due to charge transfer.

Spin-only magnetic moment μ = √[n(n+2)] BM; for n = 5 it is about 5.92 BM.

JEE Main inorganic chemistry exceptions: 25-question self-quiz

Give yourself 15 minutes and do not look at the tables. Each question has one correct option. Check the answer key below and revise the table behind every mistake.

1. Which element has the ground-state configuration [Ar]3d5 4s1? (a) Mn (b) Cr (c) Fe (d) V
2. The ground-state configuration of Cu (Z = 29) is: (a) [Ar]3d9 4s2 (b) [Ar]3d10 4s1 (c) [Ar]3d10 4s2 (d) [Ar]3d8 4s2 4p1
3. Which element has no electron in its 5s orbital in the ground state? (a) Ag (b) Mo (c) Pd (d) Ru
4. The correct order of first ionisation enthalpy in group 13 is: (a) B > Al > Ga > In > Tl (b) B > Tl > Ga > Al > In (c) B > Ga > Al > Tl > In (d) Tl > B > Ga > Al > In
5. The first ionisation enthalpy of N is higher than that of O because: (a) N is smaller (b) N has a stable half-filled 2p3 configuration (c) O is more electronegative (d) N has a higher nuclear charge
6. Which element has the most negative electron gain enthalpy? (a) F (b) Cl (c) Br (d) O
7. The correct order of negative electron gain enthalpy is: (a) O > S > Se (b) S > Se > O (c) Se > S > O (d) O > Se > S
8. The correct order of bond dissociation enthalpy of halogens is: (a) F2 > Cl2 > Br2 > I2 (b) Cl2 > Br2 > F2 > I2 (c) Cl2 > F2 > Br2 > I2 (d) I2 > Br2 > Cl2 > F2
9. The correct order of boiling points of hydrogen halides is: (a) HF > HCl > HBr > HI (b) HF > HI > HBr > HCl (c) HI > HBr > HCl > HF (d) HCl > HBr > HI > HF
10. Among group 15 hydrides, the highest boiling point is shown by: (a) NH3 (b) PH3 (c) SbH3 (d) BiH3
11. The atomic radius of Ga is smaller than that of Al mainly because of: (a) inert pair effect (b) poor shielding by 3d electrons (c) lanthanoid contraction (d) higher electronegativity of Ga
12. Zr and Hf have almost identical atomic radii because of: (a) actinoid contraction (b) lanthanoid contraction (c) inert pair effect (d) diagonal relationship
13. The strongest reducing agent in aqueous solution among alkali metals is: (a) Cs (b) K (c) Na (d) Li
14. On strong heating, LiNO3 gives: (a) LiNO2 and O2 (b) Li2O, NO2 and O2 (c) Li3N and O2 (d) Li and NO2
15. Which metal forms a superoxide on burning in excess air? (a) Li (b) Na (c) K (d) Mg
16. In the vapour phase (below about 1200 K), BeCl2 exists mainly as: (a) a linear monomer (b) a chloro-bridged dimer (c) an ionic lattice (d) a tetramer
17. The correct order of Lewis acid strength of boron halides is: (a) BF3 > BCl3 > BBr3 (b) BBr3 > BCl3 > BF3 (c) BCl3 > BF3 > BBr3 (d) all are equal
18. Boric acid, B(OH)3, is best described as: (a) a tribasic protonic acid (b) a monobasic Lewis acid (c) a dibasic protonic acid (d) a strong base
19. Which of the following does not exist? (a) PCl5 (b) NCl5 (c) SbCl5 (d) PF5
20. The more stable oxidation state of thallium is: (a) +3 (b) +1 (c) +2 (d) +5
21. PbI4 does not exist because: (a) Pb4+ is oxidising and I− is reducing (b) Pb is too large to hold four iodines (c) iodine is not electronegative enough to form bonds (d) Pb does not show the +4 state at all
22. Which pair shows a diagonal relationship? (a) Li and Na (b) Be and Al (c) B and Al (d) Mg and Ca
23. Which ion is colourless in aqueous solution? (a) Ti3+ (b) Cu2+ (c) Zn2+ (d) Fe2+
24. Which complex is square planar and diamagnetic? (a) [NiCl4]2− (b) [Ni(CN)4]2− (c) [Ni(CO)4] (d) [CoF6]3−
25. Phosphorous acid, H3PO3, is: (a) monobasic (b) dibasic (c) tribasic (d) not acidic

Answer key with one-line explanations

1. (b) Cr – One 4s electron shifts to 3d to give a stable half-filled 3d5.
2. (b) [Ar]3d10 4s1 – A fully filled 3d10 subshell is more stable.
3. (c) Pd – Palladium is [Kr]4d10 5s0.
4. (b) B > Tl > Ga > Al > In – Poor d and f shielding raises IE of Ga and Tl.
5. (b) – Half-filled 2p3 in N is extra stable; O loses a paired electron more easily.
6. (b) Cl – F's small 2p subshell repels the incoming electron more.
7. (b) S > Se > O – Oxygen's small size causes strong electron repulsion, so its value is least negative.
8. (b) Cl2 > Br2 > F2 > I2 – Lone pair repulsion weakens the short F–F bond.
9. (b) HF > HI > HBr > HCl – H-bonding makes HF highest; the rest follow molar mass.
10. (d) BiH3 – Large molar mass gives van der Waals forces stronger than H-bonding in NH3.
11. (b) – Ten 3d electrons in Ga shield poorly.
12. (b) – Poor 4f shielding cancels the expected size increase.
13. (d) Li – Very high hydration enthalpy of Li+ gives the most negative E°.
14. (b) – 4LiNO3 → 2Li2O + 4NO2 + O2, unlike other alkali nitrates which give nitrites.
15. (c) K – Large K+ stabilises the large superoxide ion; Li gives oxide, Na peroxide.
16. (b) – It becomes a linear monomer only near 1200 K.
17. (b) BBr3 > BCl3 > BF3 – Strong pπ–pπ back bonding reduces the electron deficiency of B in BF3.
18. (b) – B(OH)3 accepts OH− from water, releasing one H+; it is not a protonic acid.
19. (b) NCl5 – Nitrogen has no d orbitals, so it cannot expand its octet.
20. (b) +1 – Inert pair effect makes Tl+ more stable than Tl3+.
21. (a) – Pb4+ would oxidise I− to I2 and itself become the more stable Pb2+.
22. (b) Be and Al – Similar charge-to-radius ratio.
23. (c) Zn2+ – A d10 ion has no d–d transition, so it absorbs no visible light.
24. (b) [Ni(CN)4]2− – Strong-field CN− pairs the electrons, giving dsp2 square planar.
25. (b) Dibasic – H3PO3 has two P–OH groups and one non-ionisable P–H bond.

Final Word: what should you do next?

These JEE Main inorganic chemistry exceptions are few but appear again and again, and a few extra marks can move your percentile noticeably, as our JEE Main percentile vs rank guide shows.

• Copy the tables above into one revision sheet, with the reason next to every exception.
• Retake this quiz after three days and practise previous-year JEE Main inorganic questions.
• Check your JEE Main admit card and exam centre and the 75% board criteria well before the exam.
• If your rank falls short, plan with our guides on colleges for low JEE Main rank and NITs for a low JEE Main rank.