In the periodic table, which element has the highest first ionization energy?

• A. Chlorine (Cl)
• B. Cesium (Cs)
• C. Helium (He)
• D. Sodium (Na)

Answer: (C)

Verified step by step guidance

1. Understand that the first ionization energy is the energy required to remove one electron from a neutral atom in its gaseous state.
2. Recall that ionization energy generally increases across a period (left to right) in the periodic table because atoms have more protons, increasing the attraction to electrons.
3. Recognize that ionization energy generally decreases down a group (top to bottom) because electrons are farther from the nucleus and experience more shielding, making them easier to remove.
4. Compare the given elements: Chlorine (Cl) is in period 3, group 17; Cesium (Cs) is in period 6, group 1; Sodium (Na) is in period 3, group 1; Helium (He) is in period 1, group 18.
5. Conclude that Helium (He), being in the top right corner of the periodic table (period 1, group 18), has the highest first ionization energy among the options because it has a small atomic radius and a full outer shell, making its electrons very tightly held.