Rules for Assigning Oxidation Numbers — Prelims Strategy

Prelims Strategy

To excel in NEET questions on assigning oxidation numbers, a systematic approach is essential. First, memorize the hierarchy of rules and their exceptions. Prioritize elements like Group 1 metals ($+1$), Group 2 metals ($+2$), and Fluorine (always $-1$).

Then, focus on Hydrogen (usually $+1$, but $-1$ in metal hydrides) and Oxygen (usually $-2$, but $-1$ in peroxides, $-1/2$ in superoxides, and positive with fluorine). Second, practice extensively with a wide variety of compounds and polyatomic ions.

Include examples with complex structures, fractional oxidation states (e.g., $Fe_3O_4$, $Br_3O_8$), and compounds containing peroxide linkages (e.g., $H_2SO_5$, $CrO_5$). Third, for numerical problems, always set up an algebraic equation.

Let the unknown oxidation number be 'x' and ensure the sum of all oxidation numbers equals the overall charge of the species (zero for neutral compounds, the ion's charge for polyatomic ions). Fourth, double-check your calculations, especially when dealing with multiple atoms of the same element or negative signs.

A common trap is forgetting to multiply the oxidation number by the subscript of the element. Finally, be aware of trap options in MCQs, which often correspond to common calculation errors or overlooking exceptions (e.

g., assuming oxygen is always $-2$). If your calculated oxidation number exceeds the maximum possible for that element (e.g., $+8$ for sulfur), it's a strong indicator that you've missed an exception like a peroxide bond.