Question

Valency of \[Cl\] is \[1\] and not \[7\] . Give reasons.

Answer 1

Hint: The number of valence electrons is a measure of the number of electrons that are present in the outermost shell of an atom but valency is not the same as the number of valence electrons in an atom. It actually depends on the nature of the element being studied.

Complete answer:
Chlorine is a halogen and belongs to the seventeenth group of the modern periodic table. It is the second member of the group after fluorine and is sufficiently electronegative to be called a non-metal.
There are a total of seven valence electrons present in the outermost shell (valence shell) of the atom as the outer electronic configuration of chlorine atoms is \[n{s^2}n{p^5}\] .
Valence electrons are not the same as the valency of an atom because valency actually indicates the number of electrons contributed or gained by the particular atom in a reaction. Thus, valency actually represents the electrons needed or released by an atom to form a bond and complete its octet which may or may not be equal to the number of valence electrons.
Chlorine has a small size and experiences high nuclear charge therefore it does not have the ability to lose electrons. In order to gain its octet, it must donate seven of its electrons or accept an electron. Due to its electronegativity, chlorine gains an electron rather than losing seven of its electrons (which is a difficult process for even electropositive elements.
Therefore chlorine has its valency equal to one and not seven.

Note:
We mainly talk about the valency or valence shell configuration because the electrons present in the outermost shell are the ones that can participate in a reaction and are least affected by the nuclear pull. Also the shells before the valence shell are always completely filled.