Question

If atoms of elements have completely filled outermost shell, then their combining capacity or valency is :
(A)One
(B)Two
(C)Zero
(D)Five

Answer 1

Hint : Atoms are made of protons, neutrons and electrons. Protons and neutrons are present in the nucleus and thus they are collectively called as nucleons. Electrons are arranged in various shells present around the nucleus. In fact, the electrons present in the outermost shell of an atom greatly determines the reactivity and bonding nature of the atom.

Complete Step By Step Answer:
An atom consists of discrete energy levels called shells, around its nucleus where the electrons are located. Atomic number gives the number of electrons present in a stable and neutral atom. These electrons are arranged in various shells based on the energy and capacity of shells. The first shell which is closer to the nucleus can contain $2$ electrons, the second one can contain up to $8$ electrons and the third shell contains up to $18$ electrons and so on. The outermost shell in which electrons are present is called the valence shell and the electrons present in it are called valence electrons. These valence electrons participate in chemical bonding and determine the chemical reactivity of an atom.
Valency is the combining capacity of an atom to form chemical compounds. It is the number of electrons an atom loses, shares or gains to form a chemical bond. According to octet rule, atoms have a tendency to attain eight electrons (octet) in their outermost shell to attain stability. Atoms containing less than eight valence electrons react with other species by gaining, losing or sharing electrons to form a stable compound .Therefore, if they have a completely filled or octet configuration in the valence shell, their valency will be zero.
The right option is (C) zero.

Note :
Valence electrons are the number of electrons present in the valence shell of an atom. Valency is the number of electrons in the valence shell that can be gained, lost or shared to attain stability. It may or may not be equal to the number of valence electrons. For e.g., in Na, the number of valence electrons is equal to its valency, which is $1$ .But in the case of ${O_2}$, number of valence electrons is $6$and valency is $(8 - 6)$ $= 2$. It is an electronegative element and therefore it either shares or accepts two electrons to attain stability.