Why is the formula for milk of magnesia written MgOH2 instead of MgO2H2?

Answer 1

I think what you mean is Mg(OH)2 vs MgO2H2. This is an ionic compound because it has a metal (magnesium). In it, Mg+2 is the cation (positive ion) and OH- is the anion. Because an ionic compound must have a zero net charge, there needs to be two OH- ions to cancel the +2 charge of the magnesium. so in the first formula it is shown that there are two OH- ions, but the second one is incorrect because OH is one ion and the elements are not separately written like that in an ionic compounds formula.

hope this helps

Answer 2

The formula for milk of magnesia is written as Mg(OH)2 because magnesium hydroxide is formed when magnesium oxide (MgO) reacts with water (H2O). The hydroxide ion (OH-) from the water combines with the magnesium ion (Mg2+) to form Mg(OH)2. The subscript 2 indicates that there are two hydroxide ions associated with each magnesium ion in the compound.