Awesome question Kieran – an atom efficient chemical reaction produces no waste (i.e., all the reactants become products). The way you achieve this is through careful planning and careful choice of reactant chemicals. If you can design a suitable process the only trick is finding a catalyst that will make it work (if necessary). Hope that answers your question!
This is something that all chemists would like to do, but in reality it is very hard! For example if you use a catalyst, you woul have to recover and reuse it, as well as the solvent.. But we are getting better at being more efficient – better catalysts (chemical and bio) are improving atom efficiecy, as well as new chemical reactions that people are discovering based on the need to be more efficient..