A calcium atom is larger than a magnesium atom.
Explain why calcium reacts more vigorously than magnesium with hydrochloric acid of the same concentration.

Magnesium (Mg) and Calcium (Ca) are both in Group 2 of the periodic table. When they react, they lose their two outer shell electrons. The reactivity of Group 2 metals increases down the group. The explanation involves three factors: 1. **Atomic Radius:** A calcium atom is larger than a magnesium atom. This means its outermost electrons are in a higher energy level (shell 4 for Ca vs. shell 3 for Mg) and are further away from the positive nucleus. 2. **Shielding:** A calcium atom has more inner shells of electrons than a magnesium atom. These inner electrons "shield" the outer electrons from the full attractive force of the positive nucleus. 3. **Nuclear Attraction:** Due to the greater distance and increased shielding effect, the electrostatic force of attraction between the nucleus and the two outer electrons is weaker in a calcium atom compared to a magnesium atom. Because the outer electrons are held less strongly, less energy is required to remove them. Therefore, calcium loses its electrons more easily and reacts more vigorously than magnesium.