Arrange the following elements in order of decreasing 1st ionization energy (Cs, F, Se, P, Ga). Be clear and provide explanations.
The Correct Answer and Explanation is:
Answer (in order of decreasing 1st ionization energy):
F > P > Se > Ga > Cs
Explanation:
Ionization energy is the energy required to remove the outermost electron from a gaseous atom. The first ionization energy (IE₁) reflects how tightly an atom holds onto its valence electron. Several periodic trends influence ionization energy, primarily atomic size, nuclear charge, and electron shielding.
Across a period (left to right) on the periodic table, ionization energy increases. This is because the nuclear charge increases with each successive element, pulling electrons closer to the nucleus and making them harder to remove. Meanwhile, the added electrons go into the same energy level, so shielding doesn’t increase significantly. Thus, elements on the right side of a period have higher ionization energies.
Down a group (top to bottom), ionization energy decreases. Though nuclear charge increases, the effect is offset by increased shielding and larger atomic radii. Valence electrons are farther from the nucleus and more weakly held, making them easier to remove.
Let’s apply these ideas to the elements in question:
- Fluorine (F) is in Group 17 (halogens), Period 2. It is small, with a high effective nuclear charge and very little shielding. Therefore, it has the highest ionization energy in this list.
- Phosphorus (P) is in Period 3, Group 15. It has more shielding and a larger atomic size than F, so its ionization energy is lower.
- Selenium (Se) is in Period 4, Group 16. Though in the same group as F, it is lower in the periodic table, so it has a larger atomic radius and more shielding, resulting in a lower ionization energy than P.
- Gallium (Ga) is in Period 4, Group 13. Despite being to the left of Se, it has slightly lower ionization energy due to its electron configuration and lower effective nuclear charge.
- Cesium (Cs) is in Group 1, Period 6. It has the lowest ionization energy due to its very large size, low effective nuclear charge, and high shielding.
Thus, the correct order is: F > P > Se > Ga > Cs.
