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In NH3, what's the oxidation number of H? Should it be +3 or +1? I'm a little confused what to use.

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    $\begingroup$ What was your reason for ruling out all numbers except +1 and +3? $\endgroup$ Jul 9, 2020 at 6:06

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The oxidation number of hydrogen is mostly $1$. Unless it is in elemental state, i.e. as $\ce{H2}$, here the oxidation number of $\ce{N}$ is $-3$ and hydrogen is $+1$. Hydrogen can also take up $-1$ depending on the atom it is attached to. What you really need to lookout for is the electronegativity of the atom attached to it.

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    $\begingroup$ OS of H isn't always +1, it is only in case when H is linked with a non-metal. NaH, MgH2 are some examples of -1 OS of H. $\endgroup$ Jul 9, 2020 at 4:48

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