Explain the reactivity trend in Group II elements.

You're asking for the explanation of the reactivity trend in Group II again. Here's a concise breakdown:

1. Atomic Radius: Down Group II (from beryllium to magnesium to calcium), the number of electron shells increases, leading to a larger atomic radius.
2. Shielding Effect: The increased number of inner electron shells causes a greater shielding effect, reducing the attraction between the nucleus and the outermost valence electrons.
3. Ionization Energy: Due to the larger atomic radius and increased shielding, the valence electrons are held less tightly. This results in a decrease in ionization energy down the group, meaning less energy is required to remove an electron.
4. Reactivity: Metals react by losing electrons. Since calcium has the largest atomic radius and the lowest ionization energy among the three, it loses its valence electrons most easily, making it the most reactive. Beryllium, with the smallest atomic radius and highest ionization energy, is the least reactive.

Therefore, the reactivity order is $\boxed{\text{Calcium > Magnesium > Beryllium}}$.