H (or hydrogen), being a halogen, as an atom, has 7 valence electrons. This means that it naturally bonds with another atom of the same isotope in order to reach the desired total of valence electrons: 8.
i just did it lol but just incase, "Molecules are made of two or more atoms that are chemically bonded together. The atoms can be of the same element or of different elements. Therefore, H2, CO2, SiO2, and NH3 are all molecules."
Mg(OH)2 is the correct chemical formula because it takes two Hydroxide ions to bond with Magnesium normally it would be written with a subscript number 2 rather than the parenthesis and number 2