The oxidation number indicates the hypothetical charge of an atom in a molecule based on electronegativity rules. Hydrogen typically has an oxidation number of when bonded to non-metals (e.g., in ) but in metal hydrides where it acts as a hydride ion ( ).
Step 1: Oxidation number rules
- Alkali metals (e.g., Na) have .
- Oxygen is typically (except in peroxides).
- The sum of oxidation numbers in a neutral compound is zero.
- Hydrogen is with non-metals, with metals.
Step 2: Analyze each compound
- (A) : Sodium dihydrogen phosphate. Assign: Na: , O: , H: , P: . Neutral compound: In phosphates, P is typically . If H is : H is , not . - (B) : Sodium hydrogen sulfate. Na: , H: , S: , O: . Neutral: In sulfates, S is . If H is : H is , not . - (C) : Sodium borohydride. Na: , B: , H: . Neutral: In borohydrides, H is : B is , H is , which matches. - (D) : Elemental form, oxidation number is 0.
Step 3: Conclusion Only has hydrogen with an oxidation number of .