Given below are two statements : Statement I : The number of compounds among SO_2, SO_3, SF_4, SF_6 and H_2S in which sulphur does not obey the Octet rule is 3. Statement II : Among [H_2O, ClF_3, SF_4], [NH_3, BrF_5, SF_4], [BrF_5, ClF_3, XeF_4] and [XeF_4, ClF_3, H_2O], the number of sets in which all the molecules have one lone pair of electrons on the central atom is 1. In the light of the above statements, choose the correct answer from the options given below :
D. Statement I is false but Statement II is true
Statement I: Number of compounds in which sulphur does not follow the octet rule (from SO_2, SO_3, SF_4, SF_6 and H_2S). SO_2: The formal-charge-minimised structure O=S=O places 10 electrons around sulphur (expanded octet). SO_3: With three S=O double bonds, sulphur has 12 electrons around it (expanded octet). SF_4: 4 bond pairs + 1 lone pair = 10 electrons around S (expanded octet). SF_6: 6 bond pairs = 12 electrons around S (expanded octet). H_2S: 2 bond pairs + 2 lone pairs = 8 electrons around S (obeys the octet rule). Depending on whether formal-charge-minimised structures are used for SO_2 and SO_3, either 2 (SF_4, SF_6) or 4 (SO_2, SO_3, SF_4, SF_6) species violate the octet rule, but the number is never 3. Hence, Statement I is false. Statement II: Among the given sets, exactly one contains only molecules having one lone pair on the central atom. Number of lone pairs on the central atom, LP = valence electrons of central atom - number of bonded monovalent atoms 2 : H_2O: 6 - 2 2 = 2 lone pairs ClF_3: 7 - 3 2 = 2 lone pairs SF_4: 6 - 4 2 = 1 lone pair NH_3: 5 - 3 2 = 1 lone pair BrF_5: 7 - 5 2 = 1 lone pair XeF_4: 8 - 4 2 = 2 lone pairs Checking the sets: \ H_2O\ (2),\ ClF_3\ (2),\ SF_4\ (1)\ : not all have 1 lone pair. \ NH_3\ (1),\ BrF_5\ (1),\ SF_4\ (1)\ : all have exactly 1 lone pair. \ BrF_5\ (1),\ ClF_3\ (2),\ XeF_4\ (2)\ : not all have 1 lone pair. \ XeF_4\ (2),\ ClF_3\ (2),\ H_2O\ (2)\ : not all have 1 lone pair. Exactly one set satisfies the condition, so Statement II is true. Hence, Statement I is false but Statement II is true, which corresponds to option (4).
Related: Chemistry — Chemical Bonding and Molecular Structure · All PYQ Banks