Arrange S, O and Se in ascending order of electron affinity

The correct option is Se < S < O.

To understand this order, let us examine the concept of electron affinity and how it trends across the oxygen family (Group 16 elements: $\text{O}$, $\text{S}$, $\text{Se}$, $\text{Te}$, $\text{Po}$).

Electron affinity is the energy change that occurs when an electron is added to a neutral gaseous atom. Generally, as we move down a group in the periodic table, the atomic size increases. With the increase in atomic size, the incoming electron is added at a greater distance from the nucleus, resulting in weaker electrostatic attraction between the nucleus and the added electron. Consequently, electron affinity typically becomes less negative (decreases) down the group.

Following this general trend, we would expect the order of electron affinity to be:
$\text{O}>\text{S}>\text{Se}$

However, there is an exception between the first element (oxygen, $\text{O}$) and the second element (sulfur, $\text{S}$) of this group:
1. Oxygen has an extremely small atomic size and its valence shell is $2\text{p}$, which is very compact. Due to this high electron density, when a new electron is added to oxygen, it experiences significant inter-electronic repulsion from the electrons already present in the small $2\text{p}$ subshell.
2. Sulfur, on the other hand, has a larger atomic size and its valence shell is $3\text{p}$. The electron density is lower and the inter-electronic repulsion is much weaker. Therefore, sulfur accepts an electron more easily than oxygen, making its electron affinity more negative than that of oxygen.

Thus, the electron affinity of sulfur ($\text{S}$) is higher than that of oxygen ($\text{O}$). As we go further down to selenium ($\text{Se}$), the effect of increased atomic size dominates over electron-electron repulsion, causing the electron affinity to drop below both sulfur and oxygen.

Combining these factors:
- Sulfur ($\text{S}$) has the highest electron affinity in this set.
- Oxygen ($\text{O}$) has a lower electron affinity than sulfur due to high electron repulsion.
- Selenium ($\text{Se}$) has the lowest electron affinity due to its significantly larger size.

Therefore, arranging them in ascending order (from lowest to highest electron affinity) gives:
$\text{Se}<\text{S}<\text{O}$