Final answer:
Radium is more reactive than calcium because of its position in the periodic table, which causes its valence electrons to be held more loosely due to the larger atomic size, making it easier for radium to lose electrons and react.
Step-by-step explanation:
Radium is more reactive than calcium due to its position on the periodic table, which is the correct answer D. As one moves down a group in the periodic table, the atomic size increases because additional electron shells are added. This results in the outermost electrons being farther from the nucleus, and thus not as strongly attracted. It is easier to remove these electrons, making the element more reactive. Radium, being below calcium in Group 2 of the periodic table, has more electron shells, resulting in a lower ionization energy for its valence electrons compared to calcium.
This lower ionization energy implies that radium will more readily lose its valence electrons to form chemical bonds, which is a key aspect of its reactivity. Options A and B both mention higher atomic number and more protons, which are true for radium compared to calcium, but these factors are not the direct cause of increased reactivity. Electron affinity, as mentioned in C, typically refers to the energy change accompanying the addition of an electron to a neutral atom, which is not directly relevant to the comparison of reactivity in this case.