Question 4.8: ASSIGNING OXIDATION NUMBERS Assign oxidation numbers to each...

ASSIGNING OXIDATION NUMBERS

Assign oxidation numbers to each atom in the following substances:

(a) CdS                   (b) AlH_{3}            (c) S_{2}O_{3}^{2-}              (d) Na_{2}Cr_{2}O_{7}

STRATEGY
(a) The sulfur atom in S^{2-} has an oxidation number of -2, so Cd must be +2.

(b) H bonded to a metal has the oxidation number -1, so Al must be +3.

(c) O usually has the oxidation number -2, so S must be +2 for the anion to have a net charge of -2: for (2  S^{+2})  (3  O^{-2}), 2(+2)  + 3(-2) = -2 net charge.
(d) Na is always , +1 and oxygen is -2, so Cr must be for the compound to be neutral: for (2  Na^{+})  (2  Cr^{+6})  (7  O^{-2}), 2(+1) + 2(+6) + 7(-2) = 0 net charge.