Final answer:
Rubidium (Rb) has the smallest first ionization energy among the listed elements because it is located at the bottom left of a specified rectangle in the periodic table, coinciding with the trend that ionization energy decreases down a group.
Step-by-step explanation:
The student's question asks which of the following elements has the smallest first ionization energy: a. Rb b. Mg c. I d. As e. F. Ionization energy is the energy required to remove the outermost electron from a gaseous atom or ion.
The trend in the periodic table shows that ionization energies increase from left to right across a period and decrease from top to bottom down a group. This is because atoms become larger down a group, making it easier for electrons to be removed from the outer shell due to the greater distance from the nucleus and the increased electron shielding. Conversely, atoms become smaller across a period, which increases the nuclear charge without additional shielding from inner electrons, thus holding outer electrons more tightly and requiring higher ionization energy to remove them.
Of the options given, Rb (rubidium) is located in the bottom left of the specified rectangle in the periodic table, making it have the lowest first ionization energy. As a result, the correct answer is Rb (rubidium).