Select the elementary substance.

 - phosphine (PH3)
 - zirconium (Zr)
 - potassium nitrate (KNO3)
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 phosphine contains two symbols: P for phosphorus and H for hydrogen. So, phosphine is made of two chemical elements bonded together. Substances that are made of two or more chemical elements bonded together are compounds. So, phosphine is a compound, not an elementary substance. The chemical formula for zirconium contains one symbol: Zr. So, zirconium is made of one chemical element. Substances that are made of one chemical element are elementary substances. So, zirconium is an elementary substance. The chemical formula for potassium nitrate contains three symbols: K for potassium, N for nitrogen, and O for oxygen. So, potassium nitrate is made of three chemical elements bonded together. Substances that are made of two or more chemical elements bonded together are compounds. So, potassium nitrate is a compound, not an elementary substance.