**Zero Electron Affinity**

Electron affinity is the energy change that occurs when an atom gains an electron to form a negative ion. It represents the tendency of an atom to accept an electron. A positive electron affinity indicates that energy is released when an electron is added, while a negative electron affinity represents the energy required to add an electron.

Among the elements Helium (He), Oxygen (O), Fluorine (F), and Nitrogen (N), only Helium has zero electron affinity. This means that Helium neither releases nor requires energy when an electron is added to its outer shell.

**Helium (He) - Zero Electron Affinity**

Helium is a noble gas with a fully filled outer shell consisting of 2 electrons. It possesses the electronic configuration 1s^2. Since its outer shell is already complete, Helium does not have the tendency to gain or lose electrons. Therefore, it has zero electron affinity.

**Oxygen (O), Fluorine (F), and Nitrogen (N)**

On the other hand, Oxygen, Fluorine, and Nitrogen have electron affinities.

1. **Oxygen (O):** Oxygen has a relatively high electron affinity. It requires energy to add an electron to its outer shell, as it strives to attain a stable octet configuration. The addition of an electron to Oxygen results in the formation of the oxide ion (O^-), which has a stable electronic configuration of [He] 2s^2 2p^6.

2. **Fluorine (F):** Fluorine has the highest electron affinity among all the elements. It readily accepts an electron to achieve a stable octet configuration. The addition of an electron to Fluorine forms the fluoride ion (F^-) with a stable electronic configuration of [He] 2s^2 2p^6.

3. **Nitrogen (N):** Nitrogen has a moderate electron affinity. It requires energy to add an electron to its outer shell to achieve a stable electronic configuration. The addition of an electron to Nitrogen forms the nitride ion (N^3-) with a stable electronic configuration of [He] 2s^2 2p^6.

In summary, among Helium, Oxygen, Fluorine, and Nitrogen, only Helium has zero electron affinity. Oxygen, Fluorine, and Nitrogen possess electron affinities, with Fluorine having the highest electron affinity.

Helium has zero electron affinity because it has the maximum number of electrons it can have in it's valence shell( shell K).so, it doesn't need more electrons to get octate configuration