Which one in this pair has the larger radius? Explain

a. Calcium atom or calcium ion
b. Aluminum ion or Magnesium ion

To determine which one in each pair has a larger radius, we need to consider their atomic or ionic structures. The atomic radius is defined as half the distance between the nuclei of two atoms that are bound together.

a. Calcium atom or calcium ion:
Calcium (Ca) has 20 electrons and is located in period 4 of the periodic table. Calcium atom has a larger radius than the calcium ion because when calcium loses two electrons to form the calcium ion (Ca2+), the effective nuclear charge increases, pulling the remaining electrons closer to the nucleus, thereby reducing the atomic radius.

b. Aluminum ion or Magnesium ion:
Aluminum (Al) has 13 electrons and is located in period 3 of the periodic table. Magnesium (Mg) has 12 electrons and is also located in period 3. Both aluminum and magnesium readily lose three electrons to form Al3+ and Mg2+ ions, respectively. Comparing aluminum ion (Al3+) to magnesium ion (Mg2+), aluminum has a larger radius. This is because as aluminum loses three electrons, the effective nuclear charge increases, pulling the remaining electrons closer, resulting in a smaller atomic radius.

Therefore, in both pairs, the atom has a larger radius than the respective ion.