Redox reactions involve oxidations and reductions. Normally we think of oxidations and reductions occurring together. Electrons lost equals electrons gained. But losing and gaining are two separate actions involving different atoms. A reaction can be cut in half to show each process separately. A reaction showing either a gain or loss of electrons only is a half reaction. This PowerPoint presentation explains how to write half reactions. It illustrates the half reactions in a synthesis and a single replacement reaction. The presentation also discusses the activity series with regard to single replacement reactions. Worksheets on this topic are available free of charge on my website at www.evanschemistrycorner.com.