The correct formula for magnesium nitrate is Mg(NO₃)₂.

Quick explanation

  • Magnesium forms a Mg2+Mg^{2+}Mg2+ ion, carrying a 2+ charge.
  • Nitrate is the polyatomic ion NO3−NO_3^{-}NO3−​, carrying a 1− charge.
  • Two nitrate ions are needed to balance the 2+ charge of one magnesium ion, giving the neutral ionic compound Mg(NO₃)₂.

So, if the options look like:

  • MgNO₃
  • Mg₂NO₃
  • Mg(NO₃)₂
  • Mg₂(NO₃)₃

the correct choice is Mg(NO₃)₂.