p-Block Elements

Category: Inorganic Exception

Students assume boiling point increases monotonically with molar mass across a group's hydrides. The first-row hydrides H2O, NH3 and HF boil far higher than that trend predicts because of strong intermolecular H-bonding. H2O is the highest-boiling Group 16 hydride (NCERT Table 7.7: 373 K) and HF the highest-boiling hydrogen halide (Table 7.9: 293 K). NH3 is NOT the highest in Group 15: it boils above PH3 and AsH3 but below SbH3 and BiH3 (Table 7.2: 238.5, 185.5, 210.6, 254.6, 290 K). Any statement that places H2O or HF at the low end of their series is incorrect.

Category: Similar Terms

Noble gases are chemically inert (very high ionization enthalpy, negligible tendency to react), but this chemical stability does not carry over into thermal stability. Because the only intermolecular forces present are weak London dispersion forces, noble gases have very LOW melting and boiling points. Students who associate 'inert' or 'stable' with high thermal resistance will select a statement claiming high m.p./b.p. as correct, when in fact it is the canonical incorrect statement about noble gases.

Category: Inorganic Exception

Across period 3, first ionization enthalpy generally increases from Na to Ar, but there is a reversal at Mg/Al: IE1(Mg) > IE1(Al). Removing a 3p electron (Al) requires less energy than removing a 3s electron (Mg) because 3p orbitals are higher in energy and experience more shielding from the inner 3s electrons. Students who apply the blanket 'IE increases uniformly across a period' rule expect Al's IE to exceed Mg's, leading them to accept an incorrect statement or order.

Category: Negative Marking

NEET Assertion-Reason questions require evaluating A and R as independent statements and then determining whether R actually explains A. Students who read a plausible-sounding R alongside a correct A default to option 2 (both correct, R explains A) without separately verifying that R's specific chemical claim is factually accurate. When R is false or irrelevant, selecting option 2 instead of option 3 or 4 costs 1 negative mark.

Category: Similar Terms

A weaker bond is easier to break (lower bond dissociation energy = lower activation barrier), so a compound with a weaker bond is MORE reactive under comparable conditions, not less. Students sometimes invert this, concluding that a compound with a weaker bond is less reactive because they associate lower energy content with lower driving force. The correct relationship is: bond strength down = reactivity up.

Category: Inorganic Exception

In p-block groups, the element that deviates from the group standard oxidation state is almost always the HEAVIEST congener (due to metallic character, relativistic effects, and inert-pair tendency at high Z). Students make two opposite errors: (1) they pick the LIGHTEST member (e.g. O in Group 16) as the anomaly because oxygen is already known to have many unique properties; (2) they pick the second-heaviest (e.g. Te instead of Po) because the bottom congener is less familiar from standard syllabus coverage.

Category: Inorganic Exception

The inert pair effect is NCERT's name for the growing stability, down a group, of the oxidation state two units below the group oxidation state (Class XI Unit 11 page 1): Tl +1 over +3 (group 13), Pb +2 over +4 (group 14), Bi +3 over +5 (group 15). Students misapply it to negative oxidation states, e.g. explaining why polonium hardly shows -2. That is a different trend: NCERT states only that the stability of the -2 state decreases down Group 16 (Class XII Unit 7 page 18), alongside rising metallic character.

Category: Similar Terms

B2H6 (Group 13) is electron-deficient: it 'has too few electrons for writing its conventional Lewis structure', and its two bridge B-H-B bonds are 3-centre-2-electron bonds. GeH4 (Group 14) is electron-precise: group 14 compounds 'have the required number of electrons to write their conventional Lewis structures'. The two hydrides sit side by side in a match-the-column list and look alike, so students carry the electron-deficient label from B2H6 over to GeH4.

Category: Similar Terms

Both graphite and fullerene are non-diamond carbon allotropes with sp2-hybridised carbon and delocalised electrons, so students who remember only 'non-diamond carbon allotrope' as the distinguishing property may swap their structural labels. Graphite has a 2D layered hexagonal structure; layers slide over each other making it a dry lubricant. Fullerene (C60) is a closed cage molecule (soccer-ball shaped); its defining NEET descriptor is cage-like structure, not lubricant. Assigning cage-structure to graphite, or lubricant to fullerene, is wrong.

Category: Similar Terms

MO questions often bundle two distinct properties (bond order AND magnetic character) within a single statement about the same species. Students who have memorised the bond-order series for oxygen species (O2+: 2.5, O2: 2, O2-: 1.5, O22-: 1) confirm the bond-order part and then accept the entire multi-claim statement as correct, missing a separate magnetic-character error in the same statement. Bond order and magnetic character are derived from the same electron configuration but require independent checks: bond order from the difference of bonding vs antibonding occupancies; magnetic character from counting any residual unpaired electrons.

Category: Similar Terms

O2 having two unpaired electrons (paramagnetic) is a landmark MO result taught explicitly in NEET preparation. When O2+ appears in a question, students recall the O2 lesson and unconsciously carry over its configuration, missing that O2+ has one fewer electron. O2+ (15 electrons) has 1 unpaired electron in pi*2p, making it paramagnetic but with a different unpaired-electron count than O2. The canonical NEET trap is a statement claiming O2+ is diamagnetic, which students fail to challenge because they confuse it with O2.

Category: Inorganic Exception

For diatomics from B2 to N2 (low-Z second-period elements), the pi2p MOs lie LOWER in energy than the sigma2p MO. This is the inversion relative to the O2-type diagram where sigma2p is below pi2p. Students who learn only the O2-type ordering misplace the sigma2p electrons in C2, arriving at wrong electron occupancy for pi2p and a wrong conclusion about C2's magnetic character or number of pi bonds. C2 has 4 electrons in the two degenerate pi2p MOs (all paired, diamagnetic) precisely because pi2p is filled before sigma2p.