How to find the Oxidation Numbers for (NH4)2SO4

To find the correct oxidation state of in (NH4)2SO4 (Ammonium sulfate), and each element in the molecule, we use a few rules and some simple math.

First, since the (NH4)2SO4 molecule doesn’t have an overall charge (like NO3- or H3O+) we could say that the total of the oxidation numbers for (NH4)2SO4 will be zero since it is a neutral molecule.

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GENERAL RULES
Free elements have an oxidation state of zero (e.g. Na, Fe, H2, O2, S8).
In an ion the all Oxidation numbers must add up to the charge on the ion.
In a neutral compound all Oxidation Numbers must add up to zero.
Group 1 = +1
Group 2 = +2
Hydrogen with Non-Metals = +1
Hydrogen with Metals (or Boron) = -1
Fluorine = -1
Oxygen = -2 (except in H2O2 or with Fluorine)
Group 17(7A) = -1 except with Oxygen and other halogens lower in the group
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We know that Oxygen usually is -2 with a few exceptions. When Oxygen is in a peroxide, like H2O2 (Hydrogen peroxide), it has a charge of -1. When it is bonded to Fluorine (F) it has an oxidation number of +1.