JEE Main levelSingle correct optionDifficulty 2 of 3bn018
Assertion (A): The average oxidation number of sulfur in SX4OX6X2− is +2.5, a fraction, although no individual sulfur atom can have a fractional oxidation number.
Reason (R): In the tetrathionate structure X−X22−OX3S−S−S−SOX3X− the two terminal S atoms are +5 and the two inner S atoms (bonded only to S) are 0.
Choose the correct option.
ABoth A and R are true but R is not the correct explanation of A
BA is true but R is false
CBoth A and R are true and R is the correct explanation of A
DA is false but R is true
Answer: C
Each terminal S sits in an SOX3 group of charge −1, so x+3(−2)=−1 gives +5; the S-S bonds contribute zero, so each inner S is 0. Average =(5+0+0+5)/4=+2.5. Both statements are true and R explains why the average is fractional.