The named of this ionic compound is magnesium nitrate.
Magnesium is a group 2 alkaline earth, which forms a 2+ cation, Mg2+.
Nitrate is a polyatomic anion which has a charge of 1−. Its formula is NO−3.
Ionic compounds are named for the cation and anion present in the compound. Since Mg(NO3)2 contains the cation magnesium and anion nitrate, its name is magnesium nitrate.