It would be Magnesium = 1 atom, Oxygen = 2 atoms, Hydrogen = 2 atoms. It has 5 atoms total. There is 1 magnesium and you multiply each element by the the 2 outside the parenthesis to get 2 oxygen and 2 hydrogen.
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As metal + acid will form hydrogen gas and salt , the chemical equation is :
Mg + 2HCl --> H2 + MgCl2
