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

Last updated date: 21st Mar 2023
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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.
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.