The question asks to identify two different species that both have exactly 8 electrons in their third electron shell (n=3). One of these species should be very reactive, while the other should be very inactive.

Let's examine the electron configuration of the elements and ion mentioned in the options to determine the number of electrons in the third shell and their typical reactivity.

Electron Configurations and Third Shell Electrons

The electron configuration shows how electrons are arranged in different shells and subshells around the nucleus. The third shell corresponds to the principal quantum number n=3, which includes the 3s, 3p, and 3d subshells. For the elements in the first few periods, the 3s and 3p subshells are filled before electrons start entering the fourth shell (4s), even though the 3d subshell exists. For the species in question, we are primarily concerned with electrons in the 3s and 3p subshells.

From the table, we can see that K, Ar, Ca, and Cl⁻ all have 8 electrons in their third shell (n=3). Now let's consider their reactivity.

Reactivity Analysis of Species

Reactivity is generally related to how easily an atom or ion gains, loses, or shares electrons to achieve a stable electron configuration, typically a full valence shell (like an octet).

• K (Potassium): Potassium is an alkali metal (Group 1). It has one valence electron in its outermost shell (4s¹). Alkali metals readily lose this single valence electron to form a +1 ion ($\text{K}^+$), achieving a stable configuration like Argon. Losing just one electron requires relatively low ionization energy, making alkali metals, including Potassium, highly reactive. Potassium is considered a very reactive metal.
• Ar (Argon): Argon is a noble gas (Group 18). Its outermost shell (n=3) is completely filled with 8 electrons ($\text{3s}^2 \text{3p}^6$). This full octet configuration is very stable, meaning Argon has little tendency to gain, lose, or share electrons under normal conditions. Noble gases are known for their chemical inertness or very low reactivity. Argon is considered a very inactive species.
• Ca (Calcium): Calcium is an alkaline earth metal (Group 2). It has two valence electrons in its outermost shell (4s²). Alkaline earth metals tend to lose these two valence electrons to form a +2 ion ($\text{Ca}^{2+}$), achieving a stable configuration like Argon. Calcium is reactive, but generally less reactive than alkali metals like Potassium because removing two electrons requires more energy than removing one.
• Cl^- (Chloride ion): A chloride ion ($\text{Cl}^{-}$) is formed when a neutral chlorine atom (which has 7 valence electrons) gains one electron to achieve a stable octet configuration ($\text{3s}^2 \text{3p}^6$). Ions are charged species and have different properties than neutral atoms. The chloride ion itself has a stable electron configuration. While it participates in many reactions (e.g., forming ionic compounds or existing in solution), it is not typically described as "very reactive" or "very inactive" in the context of elemental reactivity; it's a stable species that is the result of a reactive atom gaining an electron.

Identifying the Reactive and Inactive Pair

We are looking for a pair of species that both have 8 electrons in the third shell, where one is very reactive and the other is very inactive.

• Option 1: K and Ar. Both K and Ar have 8 electrons in the third shell. K is very reactive, and Ar is very inactive. This pair fits the criteria.
• Option 2: K and Ca. Both K and Ca have 8 electrons in the third shell. K is very reactive, and Ca is reactive. Both are reactive metals; neither is very inactive. This pair does not fit the criteria.
• Option 3: Ca and Ar. Both Ca and Ar have 8 electrons in the third shell. Ca is reactive, and Ar is very inactive. While one is reactive and the other is inactive, the description "very reactive" specifically points more towards K than Ca when comparing typical elemental reactivity trends. However, based solely on 'reactive' and 'inactive', this pair could potentially be considered, but K and Ar better fit the "very reactive" and "very inactive" contrast.
• Option 4: Cl^- and Ar. Both Cl⁻ and Ar have 8 electrons in the third shell. Ar is very inactive. Cl⁻ is a stable ion, not typically classified as "very reactive" or "very inactive" like neutral atoms. This pair does not fit the criteria.

Based on the analysis, the species with 8 electrons in the third shell that are "very reactive" and "very inactive" are Potassium (K) and Argon (Ar).