Answer
a. $Ba$
b. $K$
c. $O$
d. $S$
e. $Cs$
Work Step by Step
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.