Questions & explanations
1. Which of the following is an anomalous property of oxygen compared to other elements of Group 16?
- It has the largest atomic radius.
- It can show +6 oxidation state.
- It does not have d-orbitals to expand its octet.
- It exists as a solid at room temperature.
Answer: It does not have d-orbitals to expand its octet.
Oxygen, being in the second period, lacks d-orbitals. This prevents it from showing +6 oxidation state (unlike sulphur) and limits its covalency to 4. Option A is false (oxygen has smallest radius), B is false, D is false (oxygen is a gas).
2. The shape of the sulphur dioxide (SO₂) molecule is:
- Linear
- Trigonal planar
- Bent (V-shaped)
- Pyramidal
Answer: Bent (V-shaped)
SO₂ has sp² hybridisation with one lone pair of electrons on sulphur, giving a bent (V-shaped) geometry with an O–S–O bond angle of about 119°. Linear, trigonal planar, and pyramidal shapes do not match the electron pair geometry.
3. Which of the following statements about ozone (O₃) is correct?
- It is a linear molecule.
- It is paramagnetic.
- It is a powerful oxidising agent.
- It is stable at room temperature and does not decompose.
Answer: It is a powerful oxidising agent.
Ozone is a bent molecule (angular), diamagnetic, and decomposes readily to O₂. It is a strong oxidising agent (e.g., oxidises KI to I₂). Option A (linear) is wrong; B (paramagnetic) is false; D is false because ozone is unstable.
4. The highest oxidation state shown by sulphur in its compounds is:
- +4
- -2
- +6
- +7
Answer: +6
Sulphur has available 3d orbitals, allowing it to expand its octet and achieve a maximum oxidation state of +6 (e.g., in H₂SO₄). Option A (+4) and B (-2) are common but not the highest; +7 is not possible for sulphur.
5. The correct order of thermal stability of hydrides of Group 16 elements is:
- H₂O < H₂S < H₂Se < H₂Te
- H₂O > H₂S > H₂Se > H₂Te
- H₂O > H₂Se > H₂S > H₂Te
- H₂O = H₂S = H₂Se = H₂Te
Answer: H₂O > H₂S > H₂Se > H₂Te
Thermal stability of Group 16 hydrides decreases down the group because the bond dissociation energy decreases as the size of the central atom increases. Hence, H₂O is the most stable and H₂Te the least stable.
6. Which of the following oxoacids of sulphur contains a sulphur-sulphur (S–S) bond in its structure?
- Sulphuric acid (H₂SO₄)
- Sulphurous acid (H₂SO₃)
- Thiosulphuric acid (H₂S₂O₃)
- Pyrosulphuric acid (H₂S₂O₇)
Answer: Thiosulphuric acid (H₂S₂O₃)
Thiosulphuric acid (H₂S₂O₃) has a structure in which one sulphur atom is bonded to another sulphur atom (S–S bond). In contrast, H₂SO₄, H₂SO₃, and H₂S₂O₇ contain S–O–S or S=O bonds but no direct S–S linkage.
7. The general outer electronic configuration of elements of Group 16 (chalcogens) is:
- ns² np³
- ns² np⁴
- ns² np⁵
- ns² np⁶
Answer: ns² np⁴
Group 16 elements (O, S, Se, Te, Po) have six valence electrons, giving the configuration ns² np⁴. Option A is for Group 15, C for Group 17 (halogens), and D for Group 18 (noble gases).
8. Which of the following trends is correct for Group 16 elements (O, S, Se, Te, Po)?
- Atomic size decreases down the group.
- Ionisation enthalpy increases down the group.
- Electronegativity increases down the group.
- Atomic size increases down the group.
Answer: Atomic size increases down the group.
Down a group, new electron shells are added, so atomic size increases. Ionisation enthalpy and electronegativity generally decrease down the group. Thus, only option D is correct.
9. Which of the following is NOT an allotrope of sulphur?
- Rhombic sulphur
- Monoclinic sulphur
- Plastic sulphur
- Colloidal sulphur
Answer: Colloidal sulphur
Rhombic, monoclinic, and plastic (amorphous) sulphur are well-known allotropes. Colloidal sulphur is merely a finely divided form and not a distinct allotropic modification.
10. In the Contact process for the manufacture of sulphuric acid, the catalyst used for the oxidation of SO₂ to SO₃ is:
- V₂O₅
- Fe₂O₃
- MnO₂
- Al₂O₃
Answer: V₂O₅
Vanadium pentoxide (V₂O₅) is the catalyst employed in the Contact process to speed up the reaction: 2SO₂ + O₂ ⇌ 2SO₃. Fe₂O₃, MnO₂, and Al₂O₃ are not used for this purpose.