Even though adding two electrons to O or S forms an ion with a noble gas electron configuration, the second electron affinity of both of these elements is positive. Explain.

When an atom gains or loses electrons, a charged species called an ion is created. This process is fundamental in chemistry and is driven by an atom's desire to achieve a stable electronic arrangement, typically resembling that of the noble gases.

In the case of oxygen (O) and sulfur (S), both elements have six valence electrons in their outer shell. A neutral oxygen atom becomes stable when it acquires two additional electrons, achieving a total of eight valence electrons, akin to the noble gas configuration of neon. Similarly, sulfur seeks to match the electron configuration of argon. The formation of these ions, by the addition of electrons, can either release or absorb energy, depending on whether it is the first or second electron being added.

When talking about the addition of electrons to atoms, it is essential to distinguish between exothermic and endothermic processes. An exothermic process releases energy to the surroundings, typically in the form of heat. This often occurs when the first electron is added to an atom like oxygen or sulfur because the negatively charged electron is attracted to the positively charged nucleus, releasing energy.

An endothermic process, on the other hand, requires energy to be absorbed from the surroundings for the reaction to take place. This is observed when a second electron is added to an O⁻ or S⁻ ion; the incoming electron faces repulsion from the electrons already present, and to accommodate this electron, energy must be supplied, making it endothermic.

In chemistry, electron-electron repulsion plays a critical role in determining how electrons are distributed within an atom or ion. According to the principles of electrostatics, like charges repel one another, and this is precisely what happens when attempting to add a second electron to an O⁻ or S⁻ ion.

The second electron added is repelled by the negatively charged electrons that are already occupying the outer shell. The increased repulsion raises the energy required for the addition, which is why the second electron affinity is a positive value, signifying an endothermic process. The repulsion between electrons is also an important factor in determining the shape and bonding in molecules.

Atoms of main-group elements often strive to achieve a noble gas configuration, which corresponds to a full valence electron shell – typically eight electrons, known as the octet rule. This configuration is associated with stability and non-reactivity.

While oxygen and sulfur atoms gain additional electrons to achieve this stable configuration, the process is not always energetically favorable. After gaining the first electron, these elements technically achieve a noble gas configuration when they add the second electron. However, due to the electron-electron repulsion and resulting energy requirement, the formation of a doubly-charged anion is an endothermic process and less likely to occur under normal conditions, despite the apparent 'stability' it could offer.