In atomic chemistry, the chemical reactivity of an atom mainly arises from which of the following features?

Introduction / Context:
This question examines why some atoms are very reactive while others are relatively inert. In basic atomic theory, the electrons in the outermost, or valence, shell play the key role in chemical bonding and reactivity. Atoms seek stable electron configurations, often resembling noble gases, and the presence of unpaired electrons in the valence shell creates a strong tendency to gain, lose, or share electrons. Understanding this idea is crucial for predicting chemical behaviour across the periodic table.

Given Data / Assumptions:

• Valence electrons are the electrons in the outermost shell of an atom.
• Unpaired electrons are single electrons in orbitals that are not part of an electron pair.
• The options mention features such as energy differences between shells and distances from the nucleus.
• We assume the standard Bohr and quantum mechanical models taught at school level.

Concept / Approach:
Chemical bonds form when atoms interact through their valence electrons. Atoms with completely filled valence shells, such as noble gases, are usually unreactive. Atoms with incomplete shells, especially when there are unpaired electrons, tend to form bonds to achieve a more stable arrangement. While overall energy levels and distances from the nucleus influence these processes, the direct cause of typical chemical reactivity is the presence of available unpaired valence electrons that can be shared, gained, or lost.

Step-by-Step Solution:
Step 1: Consider noble gases. Noble gases have full valence shells with paired electrons and show very low reactivity. Step 2: Consider halogens and alkali metals. Halogens have one unpaired electron in the valence shell and are highly reactive non metals. Alkali metals have one valence electron that can be easily lost, making them highly reactive metals. Step 3: Relate this to unpaired electrons. In both cases, unpaired or easily unpaired electrons in the valence shell drive reactions. Step 4: Compare with the given options. The option that directly mentions unpaired electrons in the valence shell best matches this explanation.

Verification / Alternative check:
If we examine elements in the same period, those with half filled or nearly empty valence shells are often more reactive than those with filled shells. For example, fluorine and sodium are both very reactive due to one electron short of a full shell or one electron more than a stable configuration. Neon, with a full valence shell and no unpaired electrons, is almost inert. This pattern is consistent across the periodic table and supports the idea that unpaired valence electrons are the main source of reactivity.

Why Other Options Are Wrong:
Option B: Energy differences between shells exist for all atoms and influence spectra, but they do not by themselves explain why one atom is more chemically reactive than another. Option C: The sum of potential energies of all shells is an abstract quantity and does not directly indicate reactivity in a simple way. Option D: The average distance of the outer shell from the nucleus affects ionisation energy but does not alone determine whether an atom has unpaired electrons available for bonding.

Common Pitfalls:
Students may focus too much on general energy diagrams and forget that chemical reactions involve specific electrons that can form or break bonds. Another mistake is to think that any atom with many electrons is automatically more reactive, which is not true for heavy noble gases. Keeping in mind that reactivity depends on the availability of unpaired valence electrons helps to explain both highly reactive and inert elements in a unified way.

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