Which of the following elements would have the lowest first ionization energy (IEj)?

Explanation

The first ionization energy is the energy required to remove the outermost electron from a gaseous atom. In the periodic table, ionization energy generally decreases as you move down a group because the outer electrons are farther from the nucleus and are shielded by more inner electron shells, reducing the effective nuclear charge felt by the outermost electron. Among the given options, Rubidium (Rb) is located in the fifth period and the first group, indicating that it has a larger atomic size and a lower effective nuclear charge compared to the other elements listed. Therefore, Rb has the lowest first ionization energy. In contrast, Mg, Li, and Ca are located in earlier periods, which typically have higher ionization energies due to their smaller atomic sizes and higher effective nuclear charges.