Select the elementary substance.

 - ruthenium (Ru)
 - carbon dioxide (CO2)
 - dichloromethane (CH2Cl2)
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 rhenium contains one atomic symbol: Re. So, rhenium is composed of only one chemical element. Therefore, rhenium is an elementary substance. The chemical formula for carbon dioxide contains two atomic symbols: C for carbon and O for oxygen. So, carbon dioxide is composed of two chemical elements bonded together. Because carbon dioxide is composed of multiple chemical elements bonded together, it is a compound, not an elementary substance. The chemical formula for ruthenium contains one atomic symbol: Ru. So, ruthenium is composed of only one chemical element. Therefore, ruthenium is an elementary substance. The chemical formula for dichloromethane contains three atomic symbols: C for carbon, H for hydrogen, and Cl for chlorine. So, dichloromethane is composed of three chemical elements bonded together. Because dichloromethane is composed of multiple chemical elements bonded together, it is a compound, not an elementary substance.