Understanding Electron Gain Enthalpy

Electron gain enthalpy is the energy change that occurs when an electron is added to a neutral isolated gaseous atom to form a gaseous anion. It is represented by $\Delta_e H$. When energy is released during this process, the electron gain enthalpy has a negative value, indicating that the atom has an affinity for electrons and forms a relatively stable anion. The more negative the electron gain enthalpy, the greater the tendency of an atom to accept an electron.

The question asks for the decreasing order of electron gain enthalpy with a negative sign for chalcogens (Group 16 elements: Oxygen (O), Sulfur (S), Selenium (Se), Tellurium (Te)). This means we need to arrange them from the most negative value (most energy released) to the least negative value (least energy released or potentially energy absorbed).

Trends in Chalcogens (Group 16) Electron Affinity

Generally, as we move down a group in the periodic table, the electron gain enthalpy becomes less negative. This is because:

• Atomic size increases down the group.
• The incoming electron is added to a new, larger energy shell, which is further away from the nucleus.
• The increased distance and the shielding effect of inner electrons reduce the effective nuclear charge felt by the incoming electron.
• Therefore, the attraction between the nucleus and the incoming electron decreases, leading to a less exothermic (less negative) electron gain process.

Based on this general trend, we would expect the electron gain enthalpy (with negative sign) to decrease in the order S > Se > Te.

The Anomaly of Oxygen's Electron Gain Enthalpy

However, there is an exception to the general trend for the first element of some groups, including Group 16 (Chalcogens). Oxygen (O), being the first element, is significantly smaller than Sulfur (S), Selenium (Se), and Tellurium (Te).

When an electron is added to a small atom like Oxygen, it enters the already compact 2p subshell. The existing electrons in this small volume experience significant electron-electron repulsion. This strong repulsion between the incoming electron and the electrons already present in the small atom counteracts the attraction from the nucleus.

As a result, the electron gain enthalpy of Oxygen is less negative than that of Sulfur, and in fact, it is less negative than Se and Te as well. Sulfur, being larger, accommodates the incoming electron in the 3p subshell with much less inter-electronic repulsion, making its electron gain enthalpy more negative than Oxygen's.

Determining the Correct Order of Electron Gain Enthalpy

Considering the general trend and the anomaly of Oxygen:

• Sulfur (S) has the most negative electron gain enthalpy among the chalcogens.
• Selenium (Se) and Tellurium (Te) follow the expected trend down the group after Sulfur, meaning electron gain enthalpy becomes less negative: Se > Te.
• Oxygen (O) has a less negative electron gain enthalpy than S, Se, and Te due to inter-electronic repulsion in its small size.

Combining these observations, the decreasing order of electron gain enthalpy with a negative sign is S > Se > Te > O.

Revision Table: Chalcogen Electron Gain Enthalpy

Additional Information on Group 16 Elements

• Group 16 elements are also known as Chalcogens.
• They typically have an electronic configuration of $ns^2 np^4$.
• They tend to gain two electrons to achieve a stable noble gas configuration ($ns^2 np^6$), forming anions with a charge of -2 (e.g., $O^{2-}, S^{2-}$).
• The electron gain enthalpy for adding the second electron is always positive (energy absorbed) because the incoming electron must overcome the repulsion from the already formed negative ion. Electron gain enthalpy values usually refer to the addition of the first electron unless specified otherwise.
• Other elements in Group 16 include Polonium (Po) and Livermorium (Lv).