We cannot generally produce a polymer by simply mixing a large sample of the desired monomers. Instead, additional compounds called initiators or catalysts are almost always needed to start a polymerization. One polymerization catalyst is diethylaluminum chloride, Al(C₂H₅)₂Cl. How many of each type of atom are in a molecule of this compound?

Strategy The subscripts in a formula indicate how many atoms of each type are in the molecule. The parentheses designate a group of atoms, and the subscript associated with the parentheses multiplies each atom in the group.