Select the elementary substance.

 - hydrogen peroxide (H2O2)
 - magnesium (Mg)
 - hydrazine (N2H4)
Look at the chemical formula for each substance, and count the number of symbols in the formula. Then, decide if the substance is an elementary substance. The chemical formula for hydrogen peroxide contains two symbols: H for hydrogen and O for oxygen. So, hydrogen peroxide is made of two chemical elements bonded together. Substances that are made of two or more chemical elements bonded together are compounds. So, hydrogen peroxide is a compound, not an elementary substance. The chemical formula for magnesium contains one symbol: Mg. So, magnesium is made of one chemical element. Substances that are made of one chemical element are elementary substances. So, magnesium is an elementary substance. The chemical formula for hydrazine contains two symbols: N for nitrogen and H for hydrogen. So, hydrazine is made of two chemical elements bonded together. Substances that are made of two or more chemical elements bonded together are compounds. So, hydrazine is a compound, not an elementary substance.