Which element is expected to have the lowest first ionization energy based on periodic trends?

• A. Sodium (Na)
• B. Chlorine (Cl)
• C. Cesium (Cs)
• D. Magnesium (Mg)

Answer: (C)

Verified step by step guidance

1. Recall that first ionization energy is the energy required to remove the outermost electron from a neutral atom in the gaseous state.
2. Understand the periodic trend: ionization energy generally decreases as you move down a group (column) in the periodic table because the outer electrons are farther from the nucleus and more shielded by inner electrons.
3. Also, ionization energy generally increases as you move from left to right across a period (row) due to increasing nuclear charge attracting electrons more strongly.
4. Identify the positions of the given elements: Sodium (Na), Magnesium (Mg), Chlorine (Cl), and Cesium (Cs) on the periodic table. Cesium is located much lower in Group 1 compared to Sodium, Magnesium is in Group 2, and Chlorine is in Group 17.
5. Based on these trends, the element with the lowest first ionization energy will be the one farthest down and to the left in the periodic table, which is Cesium (Cs).