Q.Which of the following gases has odour but no colour?

Among group-16 element-element single bonds, S-S is the strongest; O-O is anomalously weak.

One might expect bond energy to fall steadily down a group, but for group-16 the O-O single bond is anomalously weak. The oxygen atom is very small, so the lone pairs on the two bonded oxygen atoms repel strongly, weakening the O-O bond (about 142 kJ/mol). Sulphur is larger, so this repulsion is reduced and the S-S bond is stronger (about 226 kJ/mol). Further down, the increasing atomic …

Among O, S, Se, Te the electron affinity of oxygen is anomalously the lowest because the incoming electron enters a small, crowded 2p orbital and faces high inter-electronic repulsion. Correct option (d).

Going down group 16 the electron gain enthalpy would normally become less negative, but oxygen is an exception: its 2p orbital is so small and compact that adding an extra electron causes large electron-electron repulsion, releasing less energy than expected. As a result sulphur (not oxygen) has the most negative electron affinity, and oxygen ends up with the lowest (l …

Down group 16 the single-bond energy is highest for S-S, not O-O, because the small O atoms have strong lone-pair/lone-pair repulsion that weakens the O-O bond. Order: S-S > Se-Se > O-O > Te-Te.

Single-bond dissociation energies (approx., kJ/mol): O-O ~142, S-S ~226, Se-Se ~172, Te-Te ~126.

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Going down group 16 the central atom grows larger and less electronegative; the bonding pairs move farther out and bond-pair repulsion weakens, so the H-E-H angle shrinks toward 90deg. H2Te has the smallest angle.

Bond angles: H2O 104.5deg, H2S ~92deg, H2Se ~91deg, H2Te ~90deg.

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Oxygen shows +2 only in OF2 (F2O), because fluorine is more electronegative — option (A).

Oxygen is normally the more electronegative atom and shows negative oxidation states (usually -2, and -1 in peroxides). The ONE common exception is when it is bonded to fluorine, the only element more electronegative than oxygen.

• F2O (OF2): F is more electronegative, so each F is -1; for a neutral molecule, O must be +2. …

H2O has extensive hydrogen bonding, giving it the highest boiling point and thus the least volatility of the group.

Volatility decreases as intermolecular forces increase (higher boiling point = less volatile).

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Group 16 (O, S, Se, Te, Po) are the chalcogens.

The elements of Group 16 — oxygen, sulphur, selenium, tellurium and polonium — are collectively called chalcogens, meaning 'ore-forming', because many metal ores are oxides or sulphides.

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Checking colour and odour for each gas, only SO₂ is colourless yet distinctly odorous.

Colour and odour of the given gases:

• (a) $NO_2$: reddish-brown coloured gas — not colourless.
• (b) $SO_2$: colourless gas with a characteristic sharp, pungent, suffocating smell — matches "has odour but no colour." …