Updated 18 June 2026 · 7 min read · India
How to use this quiz
This is a quick revision quiz on the exceptions and anomalies in inorganic chemistry that show up again and again in JEE Main. Read each question, decide your answer, then tap the card to reveal the answer and a short reason. Aim to explain the why in one line, that is what separates a safe score from a top score.
Anomalous electron configurations
Q1. Why is the configuration of chromium [Ar] 3d5 4s1 and not 3d4 4s2 Tap to reveal
Answer: Because a half filled 3d5 set with 4s1 is more stable than 3d4 4s2.
Why: Exactly half filled and fully filled subshells gain extra stability from symmetric charge distribution and greater exchange energy, so one 4s electron shifts into 3d.
Q2. Why is copper [Ar] 3d10 4s1 rather than 3d9 4s2 Tap to reveal
Answer: Because a completely filled 3d10 with 4s1 is more stable than 3d9 4s2.
Why: Fully filled d subshells maximise exchange energy and symmetry, so copper adopts 3d10 4s1. The same logic explains the silver and gold anomalies.
Ionisation enthalpy exceptions
Q3. Why is the first ionisation enthalpy of boron lower than that of beryllium Tap to reveal
Answer: Because boron loses a 2p electron while beryllium loses a more tightly held 2s electron.
Why: The 2p electron in boron is higher in energy and better shielded by the filled 2s, so it is easier to remove, making boron lower despite a higher nuclear charge.
Q4. Why is the first ionisation enthalpy of oxygen lower than that of nitrogen Tap to reveal
Answer: Because nitrogen has a stable half filled 2p3 configuration.
Why: Removing an electron from nitrogen breaks a stable half filled set, which is hard. Oxygen loses a paired 2p electron and is relieved of pairing repulsion, so its value is lower.
Electron gain enthalpy and inert pair
Q5. Why is the electron gain enthalpy of chlorine more negative than that of fluorine Tap to reveal
Answer: Because the small, compact 2p subshell of fluorine causes strong electron electron repulsion.
Why: Fluorine is tiny, so the incoming electron faces high repulsion in the dense 2p shell. Chlorine is larger, the added electron is less crowded, so chlorine releases more energy.
Q6. What is the inert pair effect and one example Tap to reveal
Answer: It is the reluctance of the ns2 pair to take part in bonding down a group, for example Tl prefers +1 and Pb prefers +2.
Why: Down group 13 to 15 the ns2 electrons are held more tightly due to poor shielding by d and f electrons, so lower oxidation states (Tl+1, Pb+2, Bi+3) become more stable.
Anomalies of period 2 elements
Q7. Why does nitrogen not form pentahalides such as NCl5 while phosphorus forms PCl5 Tap to reveal
Answer: Because nitrogen has no available d orbitals, so its maximum covalency is four.
Why: Phosphorus can expand its octet using 3d orbitals to reach covalency five. Nitrogen, limited to the second shell, cannot, so NCl5 does not exist.
Q8. Why does fluorine show only the minus one oxidation state and no positive states Tap to reveal
Answer: Because it is the most electronegative element and has no d orbitals to expand its octet.
Why: With the highest electronegativity and no accessible d orbitals, fluorine cannot be oxidised by other elements, so it is restricted to minus one.
Q9. Give one example of a diagonal relationship and the reason behind it Tap to reveal
Answer: Lithium resembles magnesium because of similar charge to size ratio and polarising power.
Why: On moving across and down, size and polarising power can stay close, so pairs like Li and Mg, Be and Al, B and Si share properties. For example, lithium and magnesium both form nitrides and have covalent character in their halides.
Mixed exceptions
Q10. Why is the atomic radius of gallium slightly smaller than that of aluminium Tap to reveal
Answer: Because of poor shielding by the intervening 3d electrons in gallium.
Why: The 3d10 electrons shield the nuclear charge poorly, so the effective nuclear charge on gallium is higher, pulling its outer electrons in and offsetting the expected increase.
Q11. Why do zirconium and hafnium have almost identical atomic radii Tap to reveal
Answer: Because of the lanthanoid contraction.
Why: The steady decrease in size across the lanthanoids cancels the normal increase expected from hafnium being a period lower, so Zr and Hf end up nearly the same size and behave alike.
Q12. Why is the bond enthalpy order F2 less than Cl2 even though F is above Cl Tap to reveal
Answer: Because of strong lone pair repulsion in the small fluorine molecule.
Why: The two fluorine atoms are so small that their non bonding lone pairs repel strongly, weakening the F to F bond, so the F2 bond enthalpy is lower than that of Cl2, an important anomaly.
Score yourself
Count one mark for the correct answer and one more if you could state the reason in a sentence.
| Score (out of 24) |
What it means |
| 20 to 24 |
Strong. You can defend the why, not just the what. |
| 14 to 19 |
Good base. Revisit ionisation and inert pair logic. |
| Below 14 |
Re-read these anomalies and try again in two days. |
Revision tip. Exceptions are high yield because examiners love them. Keep a one page sheet of anomalies and read it the night before, then the morning of the exam.
Looking for another way to challenge your brain beyond chemistry. If you are a student interested in technology and problem solving, take a look at our guide to the FCRF Hackathon 2026, a cyber and digital forensics challenge for students and professionals. For more study resources, visit the 1000 Science Fair Projects home page.
This quiz is for revision support only and does not replace your textbook or teacher. Always cross check explanations with NCERT and your reference material.
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