Chemistry Regents Practice Test

Which atom in the ground state requires the least amount of energy to remove its valence electron?

1. Lithium atom

2. Rubidium atom

3. Potassium atom

4. Sodium atom

The correct answer is: Rubidium atom

The rubidium atom requires the least amount of energy to remove its valence electron because it is located further down in Group 1 of the periodic table compared to lithium, sodium, and potassium. As you move down a group, the atomic size increases due to the addition of electron shells. This increased distance between the nucleus and the outermost electron means that the valence electron is less tightly held by the positive charge of the nucleus. Additionally, the increased shielding effect from the inner electron shells reduces the effective nuclear charge experienced by the valence electron, making it easier to remove. Since rubidium has more electron shells than lithium, sodium, or potassium, the energy required to ionize a rubidium atom, which involves removing its outermost electron, is lower compared to the other elements in the group. This concept explains why rubidium is the correct choice in this context.