Helping those who are in need of help with various Chemistry related topics...
The Polyatomic Ion, which is Nitrate, has a charge of negative 1, while Magnesium has a charge of positive 2. We must take these numbers for a walk, but because we are working with a polyatomic ion, we must cover the polyatomic ion with brackets, because MgNo32 would simply be the wrong answer!
Example: Sodium Acetate
The Polyatomic Ion, which is Acetate (CH3COO), has a charge of negative 1, while Sodium has a charge of positive 1. These numbers are both 1s, so when we take them for a walk we are left with a final answer of NaCH3COO, and there is no need for a brackets around the polyatomic ion because Na is a +1; however if Sodium were replaced with Calcium, which has a charge of positive 2, the answer would end up being Ca(CH3COO)2.
Example: Copper (II) Sulfate
OH NO! WE ARE WORKING WITH A TRANSITION METAL AND A POLYATOMIC ION!!!!!!!!!
There is no need to panic! This will be easy, trust me :)
The Polyatomic Ion, which is Sulfate (SO4), has a charge of negative 2. We are working with a Transition Metal and a Polyatomic Ion, but we really do not work any differently. We are given the charge of the Transition Metal, which is Copper (positive 2). We take these numbers for a walk and we have a final answer of CuSO4, because both numbers cancel each other out. 2 - 2 = O.