In a single replacement reaction, zinc can replace copper, but copper can't replace zinc. This would not be predicted by their placement on the periodic table. Then again, lithium can replace potassium, and this is actually contradicted by their placement on the periodic table. Single replacement reactions occur in aqueous solution where the ions become hydrated. As a result, ionization energy, electronegativity, and atomic radius are not predictors of metal activity. Instead, activity is determined empirically. The resulting observations are the activity series. This PowerPoint presentation discusses the activity series and explains how to use it. Worksheets on this topic are available free of charge on my website at www.evanschemistrycorner.com.