JEE Main · practice
Inorganic chemistry exceptions quiz
25 original single-answer questions on the exceptions that JEE Main students most often mix up. Every answer explains why, and why each wrong option is wrong. This is revision practice, not an official JEE test.
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JEE Main inorganic chemistry exceptions: revision sheet
neetexamdate.in · independent practice material, not an official JEE test · not yet subject-teacher reviewed
Which has the higher first ionisation enthalpy? (Periodic trends, Easy)
- A. Boron
- B. Beryllium
- C. Both are equal
- D. Cannot be compared
Answer: B. Beryllium (2s2) has a filled 2s subshell. Boron loses a 2p electron, which is higher in energy and shielded by the 2s electrons, so it comes off more easily.
Which has the higher first ionisation enthalpy? (Periodic trends, Easy)
- A. Oxygen
- B. Nitrogen
- C. Both are equal
- D. Fluorine is lower than both
Answer: B. Nitrogen has a half-filled 2p3 configuration, which is extra stable. Oxygen has a paired electron in a 2p orbital; the repulsion makes it easier to remove, so O is lower than N.
Which element has the most negative electron gain enthalpy? (Periodic trends, Medium)
- A. Fluorine
- B. Chlorine
- C. Bromine
- D. Iodine
Answer: B. Fluorine is very small, so the added electron is strongly repelled by the electrons already in the compact 2p subshell. Chlorine's larger 3p subshell has less repulsion, so Cl releases more energy.
Among O, S, Se and Te, which has the most negative first electron gain enthalpy? (Periodic trends, Medium)
- A. O
- B. S
- C. Se
- D. Te
Answer: B. As with F and Cl, oxygen's small 2p subshell repels the incoming electron, so sulphur is more negative than oxygen. Se and Te are less negative than S.
Gallium has a smaller atomic radius than aluminium, even though it lies below it. Why? (Periodic trends, Hard)
- A. Ga has fewer shells than Al
- B. The 3d electrons shield the nuclear charge poorly
- C. Ga has a lower nuclear charge
- D. Ga loses its outer electrons more easily
Answer: B. Between Al and Ga the ten 3d electrons are added. d electrons shield poorly, so the effective nuclear charge on Ga's outer electrons is higher and the atom is not larger than Al.
Which alkali metal has the most negative standard electrode potential (M+/M)? (s-block, Medium)
- A. Cs
- B. Na
- C. Li
- D. K
Answer: C. Li has the highest ionisation enthalpy of the group but its very small Li+ ion has by far the highest hydration enthalpy. The hydration term wins, so Li is the strongest reducing agent in water.
On burning in excess air, which alkali metal forms mainly the normal oxide? (s-block, Easy)
- A. Lithium
- B. Sodium
- C. Potassium
- D. Rubidium
Answer: A. Li+ is small and polarises larger peroxide and superoxide ions, so they are unstable. Lithium gives Li2O (with a little Li2O2); sodium gives mainly Na2O2; K, Rb and Cs give superoxides.
Which alkali metal carbonate decomposes to the oxide and CO2 on moderate heating? (s-block, Medium)
- A. Li2CO3
- B. Na2CO3
- C. K2CO3
- D. Cs2CO3
Answer: A. The small Li+ ion polarises the carbonate ion strongly and Li2O has a high lattice enthalpy, so Li2CO3 decomposes. The other alkali metal carbonates are stable to much higher temperatures.
Which halide of an alkaline earth metal is predominantly covalent? (s-block, Medium)
- A. MgCl2
- B. CaCl2
- C. BeCl2
- D. BaCl2
Answer: C. Be2+ is very small with a high charge density. By Fajans' rules it strongly polarises Cl-, so BeCl2 is covalent (a polymeric chain in the solid). The others are ionic.
Which of these hydroxides is amphoteric? (s-block, Easy)
- A. Mg(OH)2
- B. Ca(OH)2
- C. Ba(OH)2
- D. Be(OH)2
Answer: D. Be(OH)2 dissolves in both acids and strong bases, giving beryllate ions. The hydroxides of Mg, Ca and Ba are basic, and basic strength increases down the group.
Which order of Lewis acid strength is correct for the boron trihalides? (p-block, Hard)
- A. BF3 > BCl3 > BBr3
- B. BBr3 > BCl3 > BF3
- C. BCl3 > BF3 > BBr3
- D. BF3 = BCl3 = BBr3
Answer: B. Electronegativity alone would predict BF3 as the strongest. But in BF3 the filled 2p orbital of F overlaps well with the empty 2p orbital of B (back-bonding), reducing electron deficiency. This overlap weakens down the group, so BBr3 is the strongest acid.
Which group 13 element is most stable in the +1 oxidation state? (p-block, Medium)
- A. B
- B. Al
- C. Ga
- D. Tl
Answer: D. The inert pair effect: the heavier the element, the less the ns2 electrons take part in bonding. Thallium is more stable as Tl+ than as Tl3+.
Which statement about group 14 oxidation states is correct? (p-block, Medium)
- A. Pb(IV) is more stable than Pb(II)
- B. Pb(II) is more stable than Pb(IV)
- C. Sn(II) is more stable than Sn(IV)
- D. C(II) is more stable than C(IV) in all compounds
Answer: B. Inert pair effect again: for lead the +2 state is more stable, so Pb(IV) compounds such as PbO2 are oxidising agents. For tin, +4 is more stable than +2, so Sn(II) is a reducing agent.
Which of these compounds does not exist? (p-block, Easy)
- A. PF5
- B. AsF5
- C. SbF5
- D. NF5
Answer: D. Nitrogen has no d orbitals in its second shell, so it cannot expand its octet to form five bonds. P, As and Sb can.
What is the basicity of phosphorous acid, H3PO3? (p-block, Medium)
- A. 1
- B. 2
- C. 3
- D. 4
Answer: B. H3PO3 has one P-H bond and two O-H groups. Only hydrogens in O-H groups are acidic, so it is dibasic. (H3PO2 is monobasic; H3PO4 is tribasic.)
Which halogen shows no positive oxidation state in its compounds? (p-block, Easy)
- A. Fluorine
- B. Chlorine
- C. Bromine
- D. Iodine
Answer: A. Fluorine is the most electronegative element and has no d orbitals, so it shows only -1 (and 0 in F2). The others show positive states in oxides, oxoacids and interhalogens.
What is the ground-state electron configuration of chromium? (d-block, Easy)
- A. [Ar] 3d4 4s2
- B. [Ar] 3d5 4s1
- C. [Ar] 3d6 4s0
- D. [Ar] 3d3 4s2
Answer: B. A half-filled d subshell with a half-filled s subshell (3d5 4s1) is more stable than 3d4 4s2, because of exchange energy and the small 3d-4s gap. Copper likewise is 3d10 4s1.
Which of these is not counted as a transition element by the IUPAC definition? (d-block, Medium)
- A. Fe
- B. Cu
- C. Ni
- D. Zn
Answer: D. IUPAC counts an element as transition if its atom or a common ion has an incomplete d subshell. Zn (3d10 4s2) and Zn2+ (3d10) are both full. Cu2+ is 3d9, so Cu still counts.
Which 3d metal has a positive standard electrode potential for M2+/M? (d-block, Medium)
- A. Fe
- B. Zn
- C. Cu
- D. Ni
Answer: C. Copper is +0.34 V: the enthalpy of atomisation is high and the hydration enthalpy of Cu2+ is not large enough to compensate. So Cu does not liberate hydrogen from dilute acids.
Zr and Hf have almost the same atomic radii. The cause is: (d-block, Medium)
- A. Inert pair effect
- B. Lanthanide contraction
- C. Diagonal relationship
- D. Jahn-Teller distortion
Answer: B. The 4f electrons filled across the lanthanides shield poorly, so the 5d series (Hf) is smaller than expected and ends up close in size to the 4d series (Zr).
Which complex is square planar and diamagnetic? (Coordination, Hard)
- A. [NiCl4]2-
- B. [Ni(CO)4]
- C. [Ni(CN)4]2-
- D. [Zn(NH3)4]2+
Answer: C. CN- is a strong-field ligand, so in Ni2+ (d8) the electrons pair up, leaving one empty d orbital used in dsp2 hybridisation: square planar and diamagnetic. [NiCl4]2- is tetrahedral and paramagnetic. [Ni(CO)4] is tetrahedral (Ni(0), d10) and diamagnetic. [Zn(NH3)4]2+ is tetrahedral.
Which of these oxides of nitrogen is neutral? (Bonding and oxides, Easy)
- A. N2O
- B. N2O3
- C. N2O5
- D. NO2
Answer: A. N2O (and NO) are neutral oxides: they do not react with water to give acids or with bases to give salts. N2O3, N2O5 and NO2 are acidic.
Molecular orbital theory correctly predicts that O2 is: (Bonding and oxides, Medium)
- A. Diamagnetic with bond order 2
- B. Paramagnetic with bond order 2
- C. Paramagnetic with bond order 3
- D. Diamagnetic with bond order 3
Answer: B. O2 has 12 valence electrons; the last two occupy the degenerate pi* orbitals singly, giving two unpaired electrons (paramagnetic). Bond order = (8 - 4)/2 = 2. A simple Lewis structure wrongly suggests all electrons are paired.
Which is the weakest acid in water? (Bonding and oxides, Easy)
- A. HF
- B. HCl
- C. HBr
- D. HI
Answer: A. HF is the weakest because the H-F bond is very strong and F- is strongly hydrogen-bonded. Acid strength rises down the group: HF < HCl < HBr < HI, which is the reverse of what electronegativity alone suggests.
Which halogen molecule has the highest bond dissociation enthalpy? (Bonding and oxides, Hard)
- A. F2
- B. Cl2
- C. Br2
- D. I2
Answer: B. The order is Cl2 > Br2 > F2 > I2. F2 is unusually weak because the lone pairs on two very small F atoms repel strongly across the short F-F bond.