Fe(no3)2 not sure how to get the oxidation numbers of all elements

The oxidation number of oxgyen is -2. In the compound, the total charge of oxygen is -12. Fe has a charge of +2. To make the sum equal to zero, N has to have a charge of +5. -12+2+2x=0; x= +5. 

Because electrons have negligible mass, to account for the mass of the isotope, there must be 16 neutrons, 16 neutrally charged, massive, fundamental particles present in the sulfur nucleus. and thus finally, if we have the 32s2− ion, there are 18 electrons, 16 protons, and 16 neutrons.

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