 If you wanted to make a cake you would add flour, eggs, water, baking soda and your favourite flavours. You may add more ingredients but the important idea here is that all the ingredients will be included in the final product, your cake. Following on the same idea, reactions with a high atom economy is one where all of the atoms in the reactants are included in the final desired product. Such a reaction would have little, if any, waste produced. According to law of conservation of mass, no atoms are created or destroyed in a reaction. The atoms from the reactants are simply rearranged to form products. So why not maximise a certain reaction so that no atoms are wasted as side products? Going back to the cake example, it wouldn't be a good idea to add all those ingredients but then find that one of them is not baked into your final cake. Here's a quick challenge for you. When an alkene such as hex2ene is hydrogenated in the presence of a metal catalyst, what is the product? Pause, think and resume when ready. The answer is hexane. This hydrogenation reaction has a 100% atom economy. The final product, hexane, has all the atoms from the reactants, hex2ene and hydrogen. No atoms are wasted as unwanted byproducts. Furthermore, the use of a catalyst makes this process very efficient and the catalyst can be reused.