Element being oxidized.....

Oxidation is the increase in oxidation state. Reduction is the decrease in oxidation state. In some texts oxidation and reduction are defined in terms of electron "gain" or "loss", but in many cases there is actually no loss or gain of electrons, its just on paper. But there will always be oxidation state changes in redox reactions, making that the better indicator of oxidation and reduction.

Start by writing the balanced net ionic equation, and assigning oxidation states to the elements.

...+2......-3+1......... +1-2.........0 ........ +5-2......... +1

4Fe2+ + NH4^+ + 3H2O --> 4Fe(s) + NO3^- + 10H+

You can easily see that ferrous ion has been reduced and nitrogen in ammonia has been oxidized.

For an element to be oxidized, the oxidation number must increase. It is obvious that iron decreased from +2 to 0. So, the oxidation number of nitrogen must have increased.

In NH4+1, H is +1. So N is -3. In NO3-1, O is -2.

3 * -2 = -6

N + -6 = -1

N = +5

The oxidation number of nitrogen has increased from +1 to + 5.

(NH4)^+ ; oxidation number of H = +1

4(+1) + 1(N) = +1 ; N = -3 oxidation number

(NO3)^- ; oxidation number of O usually -2

1(N + 3(-2) = -1 ; N = +5

Thus, N goes from a -3 oxidation number to a +5. That involves the loss of electrons, which

is oxidation. N in NH4^+ is oxidized.