In atomic and chemical reactivity, the chemical reactivity of an atom mainly arises from which feature of its valence shell?

Introduction / Context:
This question focuses on what makes atoms chemically reactive. Chemical reactions occur when atoms gain, lose, or share electrons, usually those in the outermost shell (the valence shell). The presence or absence of unpaired electrons in this valence shell plays a major role in determining whether an atom is likely to form bonds. Atoms with filled valence shells, such as noble gases, are generally unreactive, while atoms with one or more unpaired electrons tend to be reactive as they seek more stable electron configurations through bonding.

Given Data / Assumptions:
- The options describe different possible factors related to electron shells and energy levels.
- The focus is on the valence shell, which contains electrons involved in bonding.
- It is assumed that you understand the concept of valence electrons and the difference between paired and unpaired electrons.
- The goal is to identify the feature that most directly drives chemical reactivity.

Concept / Approach:
Chemical bonds form when atoms share or transfer electrons to achieve more stable configurations, often resembling noble gas electron arrangements. Valence electrons are the ones that participate in bonding. If all valence electrons are paired in stable orbitals, as in noble gases, the atom has little tendency to react. In contrast, unpaired valence electrons represent incomplete shells or subshells and drive atoms to form bonds that allow them to pair up and lower their energy. While factors like potential energy levels and distance from the nucleus influence reactivity indirectly, the most direct and practical indicator of chemical reactivity is the presence of unpaired electrons in the valence shell.

Step-by-Step Solution:
Step 1: Recall that valence electrons are the electrons in the outermost shell that participate in chemical bonding. Step 2: Recognise that noble gases have completely filled valence shells with all electrons paired and are very unreactive. Step 3: Observe that elements such as halogens and alkali metals have unpaired valence electrons and are highly reactive as they seek to gain or lose electrons to achieve filled shells. Step 4: Understand that while overall potential energy and electron shell distances affect energy levels, it is specifically unpaired electrons that create opportunities for new bonds. Step 5: Conclude that the existence of unpaired electrons in the valence shell is the main reason for the chemical reactivity of an atom.

Verification / Alternative check:
Consider specific examples. Neon and argon have full valence shells with no unpaired electrons and are chemically inert under ordinary conditions. Sodium has one unpaired electron in its outer shell and is very reactive, readily losing that electron to form Na+. Chlorine has seven valence electrons with one unpaired, making it reactive and eager to gain one electron to complete its octet. Transition metals often have several unpaired d electrons and show a variety of oxidation states and bonding behaviours. This pattern across the periodic table supports the idea that unpaired valence electrons are closely linked to reactivity.

Why Other Options Are Wrong:
The potential energy of the valence shell as a whole is a broad concept and does not directly capture why some atoms react more than others; it is the specific arrangement and pairing of electrons that matters. The average distance of the outermost shell from the nucleus influences ionisation energy and atomic size but does not by itself dictate reactivity without considering valence electron configuration. The sum of potential energies of all electron shells is even more general and is not used as a practical criterion for chemical reactivity. These options are more abstract and do not pinpoint the factor that chemists actually use to predict reactivity, which is the presence of unpaired valence electrons.

Common Pitfalls:
Students sometimes overemphasise atomic radius or ionisation energy without connecting these properties back to valence electron configurations. Others may think of energy in a vague way, assuming that higher energy always means higher reactivity without considering how electrons are arranged. To avoid these pitfalls, remember that electron pairing in the valence shell is a concrete, visualisable criterion: unpaired electrons signal reactivity, while fully paired electrons indicate relative inertness. This is why noble gases with filled shells are inert and why atoms with half filled or nearly filled shells often show strong chemical activity.

Final Answer:
The chemical reactivity of an atom mainly arises from the existence of unpaired electrons in the valence shell.