The atom economy of a reaction is defined as the percentage of atoms from the reactants that are incorporated into the desired products. It is a measure of how efficient a reaction is in terms of atom utilization.
The atom economy of ammonia is 100% because the only reactants are nitrogen and hydrogen, and the only product is ammonia. All of the nitrogen and hydrogen atoms that are used in the reactants are also present in the ammonia product. There is no waste.
This is in contrast to many other reactions, which have atom economies that are less than 100%. For example, the atom economy of the combustion of methane is only 50%, because only half of the atoms in the methane molecule are incorporated into the products (carbon dioxide and water). The other half of the atoms are released into the atmosphere as waste products (nitrogen and oxygen).
