The chemical properties and reactivity of an element mainly depend on which of the following features of its atomic structure?

Introduction / Context:
This question tests your understanding of how atomic structure controls the chemical behaviour of elements. In periodic table and bonding chapters, you repeatedly see that valence electrons, that is, electrons in the outermost shell, determine how an atom will bond, what type of compounds it will form and how reactive it is. Recognising this relationship is crucial for predicting trends and reactions in chemistry.

Given Data / Assumptions:

• The question asks which atomic property chemical properties depend on.
• Options mention isotopes, mass number, neutrons, outermost electrons and total nucleons.
• We assume neutral atoms in ground state and basic school level chemistry.

Concept / Approach:
Chemical properties are determined by how atoms gain, lose or share electrons to achieve stable electronic configurations. These processes mainly involve the electrons in the highest energy shell, called valence electrons. The number of valence electrons decides whether an element behaves as a metal, non metal or noble gas and whether it tends to form ionic or covalent bonds. Mass number, number of neutrons and isotopic composition have relatively little effect on ordinary chemical reactions, though they do affect nuclear properties and atomic mass.

Step-by-Step Solution:
Step 1: Recall that in the Bohr model and modern atomic theory, electrons occupy shells or energy levels around the nucleus. Step 2: Electrons present in the outermost shell are called valence electrons. Step 3: During chemical reactions, atoms typically lose, gain or share these valence electrons to reach a stable configuration, often like that of the nearest noble gas. Step 4: The number of valence electrons therefore determines the valency, types of bonds and general reactivity of the element. Step 5: Isotopes of an element have the same number of protons and electrons but different numbers of neutrons; they show almost identical chemical behaviour, which proves that neutrons and mass number are not the primary determinants of chemical properties. Step 6: Total nucleons (protons plus neutrons) define mass number and nuclear properties but do not control how atoms bond in typical chemical reactions. Step 7: Hence, the correct choice is the number of electrons in the outermost shell of the element.

Verification / Alternative check:
You can verify this principle by looking at elements in the same group of the periodic table. For example, all alkali metals in group 1 have one electron in their outermost shell and show similar chemical properties such as forming +1 ions and reacting vigorously with water. Similarly, all halogens in group 17 have seven valence electrons and form similar types of compounds. Their isotopes, which differ in neutron number, do not differ much in chemical behaviour. This pattern confirms that valence electrons are the key factor.

Why Other Options Are Wrong:
The number of isotopes only tells how many naturally occurring varieties of an element exist and has little effect on its basic chemistry. Mass number and total neutrons influence nuclear stability and atomic mass but do not change the way electrons are arranged in shells for a given element. Total protons plus neutrons is again just the mass number. Only the arrangement and count of electrons, especially in the outermost shell, controls chemical properties and reaction patterns.

Common Pitfalls:
Students sometimes confuse physical properties like atomic mass or density with chemical properties. Although mass can slightly affect reaction rates, it does not fundamentally change how the element reacts. Another pitfall is to think that more massive atoms are always more reactive, which is not always true. Focusing on valence electrons provides a cleaner and more reliable way to understand chemical behaviour across the periodic table.

Final Answer:
The chemical properties of an element mainly depend on the number of electrons in the outermost shell of the element.