Chapter 5 - Electron Configurations and the Periodic Table - Questions for Review and Thought - Topical Questions - Page 240b,: 60a

$Cs^{+}$ and $Se^{2-}$ are likely to be found in an ionic compound

Potassium does not form $K^{2+}$ ion. Cs forms a $Cs^{+}$ ion Aluminum does not form a $al^{4+}$ ion. Florine does not form a $F^{2-}$ ion. Selenium forms a $Se^{2-}$ ion.