Has the lowest first-ionization energy — Atomic Structure Chemistry Question

STEPS:

1. Define First^-Ionization Energy: First-ionization energy is the amount of energy required to remove the most loosely bound electron from an atom in its gaseous state.
2. Recall Periodic Trends: Generally, first-ionization energy increases as you move from left to right across a period (due to increasing effective nuclear charge) and decreases as you move down a group (due to increased electron shielding and a greater distance between the nucleus and the valence electrons).
3. Locate the Elements on the Periodic Table: Using the provided periodic table, identify the positions of the choices:
* $Cs$ (Cesium): Period 6, Group 1.
* $Ag$ (Silver): Period 5, Group 11.
* $Pb$ (Lead): Period 6, Group 14.
* $Br$ (Bromine): Period 4, Group 17.
* $Se$ (Selenium): Period 4, Group 16.
4. Identify the Extremes: The lowest ionization energy is typically found in the bottom-left corner of the periodic table, where atoms have the largest radii and their valence electrons are furthest from the nucleus.
5. Compare Positions: Cesium ($Cs$) is located furthest to the left (Group 1) and is tied for the furthest down (Period 6). Its single valence electron is in the 6s orbital, very far from the nucleus and heavily shielded by 54 inner electrons, making it exceptionally easy to remove compared to the other elements listed.

WHY_OTHERS_WRONG:

• $Br$ (D) and $Se$ (E): These are nonmetals located in Period 4. Being higher up and further to the right than Cesium, they have smaller atomic radii and much higher effective nuclear charges, resulting in very high ionization energies.
• $Ag$ (B): While it is a metal, Silver is in Period 5 and in Group 11. It is higher up and further to the right than Cesium, meaning its valence electrons are more tightly held by the nucleus.
• $Pb$ (C): Lead is in the same period as Cesium (Period 6), but it is in Group 14. Because it has more protons (82 vs. 55), it has a much higher effective nuclear charge, which pulls its electrons in more tightly and requires more energy to remove them than Cesium.