a. Which of the following cations is least likely to form: \(\mathrm{Sr}^{2+}, \mathrm{Al}^{3+}, \mathrm{K}^{2+} ?\) b. Which of the following anions is least likely to form: \(\mathrm{I}^{-}, \mathrm{Cl}^{-}, \mathrm{O}^{2-?}\) ?

Cations are positively charged ions that form when an atom loses one or more electrons. This loss results in a greater number of protons than electrons. The process of forming cations is typically tied to an element's desire to achieve a stable electron configuration, often mirroring the closest noble gas.

For instance:

• Strontium (Sr) loses two electrons to form \(\text{Sr}^{2+}\), resembling the electron configuration of xenon (Xe).
• Aluminum (Al) loses three electrons to form \(\text{Al}^{3+}\), achieving a stable configuration similar to neon (Ne).
• Potassium (K) primarily loses one electron to form \(\text{K}^+\), mimicking the electron structure of argon (Ar).

Given these tendencies, forming \(\text{K}^{2+}\) would be energetically unfavorable and unstable, making it the least likely cation among \(\text{Sr}^{2+}, \text{Al}^{3+}, \text{K}^{2+}\).

Anions are negatively charged ions created when an atom gains one or more electrons. This gain leads to a greater number of electrons than protons. Much like cations, the formation of anions is driven by the pursuit of a stable electron configuration, often resembling that of a noble gas.

For example:

• Iodine (I) commonly gains one electron to form \(\text{I}^-\), giving it a configuration like xenon (Xe).
• Chlorine (Cl) also gains one electron to form \(\text{Cl}^-\), achieving an electron arrangement similar to argon (Ar).
• Oxygen (O) typically gains two electrons to form \(\text{O}^{2-}\), aligning its configuration with neon (Ne).

Among \(\text{I}^- , \text{Cl}^- , \text{O}^{2-}\), \(\text{O}^{2-}\) is less probable due to the higher energy requirement for oxygen to gain two electrons compared to just one.

Electron configuration describes the distribution of electrons in an atom’s orbitals. It plays a key role in determining how an atom will interact with other atoms, including its propensity to form cations or anions.

Atoms aim to accomplish the electron configuration of the nearest noble gas, as achieving such a state signifies maximum stability. This often means:

• Losing electrons to form cations when fewer electrons are needed to reach stability.
• Gaining electrons to form anions when more energy is released by acquiring electrons.

Notably, potassium (K), which lies in Group 1 of the periodic table, loses one electron to satisfy its octet rule by adopting the electron configuration of argon (Ar). In contrast, elements like strontium (Sr) and aluminum (Al) form \(\text{Sr}^{2+}\) and \(\text{Al}^{3+}\) by losing two and three electrons respectively, mitigating their electron count to achieve a stable state similar to a noble gas.