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When the oxidation number of an element decreases over the course of a reaction, that element has been.... A ionized. B. Oxidized. C.redoxed, D.reduced

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Answer:

D. Reduced  

Step-by-step explanation:

Reduction is a decrease in oxidation number.

You have probably learned OIL RIG, where the RIG means Reduction Is Gain of electrons.

If an atom has an oxidation number of +4 and it gains two electrons, its oxidation number decreases to +2.

A is wrong, because in the electrolysis of H₂O one product of the reaction is H₂, and the H is not ionic.

C is wrong, because "redoxed" is not a word.