Question

Explain why the first electron affinity of sulfur is $200\mathrm{kJ}/\mathrm{mol}$ but the second electron affinity is $-649\mathrm{kJ}/\mathrm{mol}$.

Short answer

Sulfur's first electron affinity is positive as energy is released, but the second is negative as energy is needed to overcome repulsion in an already charged ion.

Step-by-step solution

Understanding Electron Affinity

Electron affinity refers to the energy change when an electron is added to a neutral atom in the gaseous state. The first electron affinity is usually a positive value because energy is released when an electron is added, indicating that the atom has gained an electron and achieved a more stable configuration.

Evaluating Sulfur's First Electron Affinity

For sulfur, the first electron affinity is $200\text{ kJ/mol}$, which means energy is released when the first electron is added to neutral sulfur. The added electron fills one of the vacancies in the 3p orbital, making the resulting $S^{-}$ ion more stable.

Explaining Negative Second Electron Affinity

The second electron affinity has a negative value $-649\text{ kJ/mol}$. This indicates that energy is required to add a second electron to the already negatively charged $S^{-}$ ion, overcoming the electron-electron repulsion, thereby making the $S^{2-}$ ion. Since the ion is already negatively charged, it resists the addition of another electron, thus requiring additional energy input.

Conclusion

In summary, the first electron affinity of sulfur is positive, indicating energy release and stability upon gaining one electron. In contrast, the second electron affinity is negative because it requires energy to force an additional electron into an already negatively charged ion.

Key concepts

Sulfur Electron Affinity

When we talk about the sulfur electron affinity, we're discussing how sulfur atoms interact with electrons. Electron affinity refers to the change in energy when an electron is added to an atom. For sulfur, the first electron affinity is equal to $200extkJ/mol$. This positive value tells us that when a single electron is added to a neutral sulfur atom, energy is released.
The sulfur atom reaches a more stable state by completing its electron shell partially, making it more energetically favorable than before. When an electron integrates into the sulfur atom’s 3p orbital, it forms a stable $S^{-}$ ion. A positive electron affinity indicates this process is favorable due to the stability gained by the addition of an electron.

Energy Change

Energy change is a vital concept in chemistry and it plays a significant role in electron affinity. The energy change refers to the amount of energy that is either absorbed or released when a chemical process occurs. When talking about electron affinity, this process usually involves the release of energy – meaning the atom becomes more stable.
For sulfur, the addition of the first electron results in energy release since sulfur becomes more stable, as indicated by the energy value of $200extkJ/mol$. Energy release suggests that nature favors stability, and a lowered energy state corresponds to a more stable and happy atom or ion. Such changes are pivotal to understanding chemical behavior and reactions.

Electron-Electron Repulsion

Electron-electron repulsion is an important phenomenon that occurs when electrons are added to an atoms. Electrons carry a negative charge, and thus they naturally repel each other.
When a second electron is introduced to an already negatively charged ion like the $S^{-}$ ion, this repulsion becomes a critical factor. The existing electron cloud repels the incoming electron making it more challenging to add a second one.
This is why the second electron affinity is negative (-649 kJ/mol). It reflects the additional energy input required to forcefully add another electron to an already crowded electron field causing a further negatively charged $S^{2-}$ ion.

Second Electron Affinity

The concept of second electron affinity helps us understand why adding additional electrons can sometimes require more energy. When an $S^{-}$ ion already hosts an additional electron, adding yet another one seems unfavorable due to the already established negative charge.
The second electron affinity of sulfur is -649 kJ/mol, meaning energy is required, rather than released, to add this electron. This is primarily because of the electron-electron repulsion present in the $S^{-}$ ion.
By requiring more energy to add a second electron, sulfur becomes a $S^{2-}$ ion. This energy investment is necessary to overcome the natural repulsion between electrons, making the additional negative charge possible.