Concept:Valency is the combining capacity of an atom, determined by the number of electrons it can gain, lose, or share to attain a stable electron configuration.
This depends solely on the outermost shell electrons.
Explanation:The outermost shell of an atom is called the valence shell.
The number of electrons in this shell directly defines the valency.
For example, sodium has
1 valence electron, so its valency is
1.
Chlorine has
7 valence electrons, so it needs
1 more to complete its octet, giving a valency of
1.
The total number of protons (atomic number) is fixed and does not affect chemical bonding.
Mass number (protons + neutrons) and neutron count are related to isotopes, not combining capacity.
Thus, valency depends only on the number of electrons in the outermost shell.
Answer:The correct option is D: total number of electrons in the outermost shell of an atom.