An atom that becomes more energetically stable when it loses a single electron is typically one that is in Group 1 of the periodic table, as losing one electron allows it to achieve a stable electron configuration, similar to that of the nearest noble gas.
In this case, among the options provided:
- Magnesium (Mg): It is in Group 2 and would lose two electrons to become stable.
- Cesium (Cs): It is in Group 1 and losing one electron would indeed make it more stable (it would become Cs⁺).
- Fluorine (F): It gains an electron to achieve stability, not loses one.
- Boron (B): It doesn't achieve stability by losing a single electron; it typically shares electrons in covalent bonding.
Therefore, the correct answer is Cesium (Cs).