The oxidation number of an element in its free, uncombined state is
Correct answer: D. zero
- A. plus 1
- B. equal to its group number
- C. minus 1
- D. zero
Explanation
An uncombined element has not gained or lost any electrons, so its oxidation number is zero, and this holds for atoms such as helium and for molecules such as O2, P4 and S8 alike. This rule is the starting point for every oxidation number calculation and for recognising which species has been oxidised in a reaction. The group number gives the usual oxidation state in compounds, not in the free element.
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About Oxidation and Reduction
Oxidation involves loss of electrons, increase in oxidation number or addition of oxygen, while reduction involves electron gain, decrease in oxidation number or removal of oxygen. The topic covers oxidising and reducing agents, oxidation-number changes, redox equations and the distinction between oxidation at the anode and reduction at the cathode.
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