Electron Affinity and Energy Release of Elements

Definition: Electron affinity refers to the amount of energy released when an electron is added to a neutral atom in the gaseous state. This process is indicative of how much an atom wants to gain an electron, leading to the formation of a negative ion.
Energy Release Context: The more energy released during the gain of an electron, the more favorable the electron addition. This energy release occurs due to the attractive force between the added electron and the nucleus of the atom.
Elements in Question:
- Sodium (Na)
- Helium (He)
- Carbon (C)
- Chlorine (Cl)

Sodium (Na):
- Atomic number: 11
- Tends to lose one electron to attain a stable electronic configuration resembling that of the nearest noble gas (Neon). Electron affinity is relatively low compared to nonmetals.
Helium (He):
- Atomic number: 2
- Helium is a noble gas with a full outer shell (1s²), making it unlikely to gain an electron and thus has a very low electron affinity since it does not want to form a negative ion.
Carbon (C):
- Atomic number: 6
- Carbon can either gain or lose electrons. When gaining four electrons, it would obtain a full outer shell via the formation of a carbide ion (C^{4-}), but generally, the energy released when gaining a single electron is moderate.
Chlorine (Cl):
- Atomic number: 17
- Chlorine has a high tendency to gain an electron to achieve a full outer shell configuration (similar to Argon). This process is highly exothermic, resulting in significant energy release during electron gain.

Among Sodium, Helium, Carbon, and Chlorine, Chlorine (Cl) will release the most energy upon gaining an electron due to its high electronegativity and desire to complete its valence shell.
The trend shows that nonmetals, particularly halogens like Chlorine, demonstrate significant electron affinity compared to alkali metals and noble gases.