## Chemistry 9th Edition

a. $Ba$ b. $K$ c. $O$ d. $S$ e. $Cs$
a. Ionization energy generally increases as we go across a period and decreases with higher principal energy levels. Therefore, $Ba$ has the lowest ionization energy. b. Ionization energy generally increases as we go across a period and decreases with higher principal energy levels. Therefore, $K$ has the lowest ionization energy. c. Ionization energy is lower when there is only one electron left in a subshell or when there is only one pair of electrons in a subshell. Therefore, $O$ has the lowest ionization energy. d. Ionization energy is lower when there is only one electron left in a subshell or when there is only one pair of electrons in a subshell. Therefore, $S$ has the lowest ionization energy. e. Ionization energy generally increases as we go across a period and decreases with higher principal energy levels. Therefore, $Cs$ has the lowest ionization energy.