where did u get the positive to charge for magnesium and for chlorine -1

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That's the oxidation numbers for both elements. If you look at magnesium on the Periodic table, You will see that Magnesium is in the 2nd column which means it has a +2 charge which means it wants to lose 2 electrons. Chlorine on the other hand needs to gain one electron because its right next to the Noble gases (which have an octet).   Google Oxidation numbers if this don't make sense.