Question

What is valency shell in atom

Answer 1

The electrons in the outermost occupied shell (or shells) determine the chemical properties of the atom; it is called the valence shell.

The valence shell is the outermost shell of an atom. Valence electrons in non-transition metal elements reside in this shell. Such elements with complete valence shells (noble gases) are the most chemically non-reactive, while those with only one electron in their valence shells (alkali metals) or just missing one electron from having a complete shell (halogens) are the most reactive.
However, this terminology is somewhat misleading in the case of transition metals. In these elements, a valence electron can also be in an inner shell. Thus, the electrons that determine how an atom reacts chemically are those that travel farthest from the nucleus, that is, those with the highest energy, and not necessarily in the valence shell.